Better boot fail error reporting

src/Umbraco.Core/CoreRuntime.cs

@@ -105,6 +105,7 @@ namespace Umbraco.Core
                 {
                     _state.Level = RuntimeLevel.BootFailed;
                     var bfe = e as BootFailedException ?? new BootFailedException("Boot failed.", e);
+                    _state.BootFailedException = bfe;
                     bootTimer.Fail(exception: bfe); // be sure to log the exception - even if we repeat ourselves
 
                     // throwing here can cause w3wp to hard-crash and we want to avoid it.

src/Umbraco.Core/Exceptions/BootFailedException.cs

@@ -7,6 +7,11 @@ namespace Umbraco.Core.Exceptions
     /// </summary>
     public class BootFailedException : Exception
     {
+        /// <summary>
+        /// Defines the default boot failed exception message.
+        /// </summary>
+        public const string DefaultMessage = "Boot failed: Umbraco cannot run. Sad. See Umbraco's log file for more details.";
+
         /// <summary>
         /// Initializes a new instance of the <see cref="Exception"/> class with a specified error message.
         /// </summary>

src/Umbraco.Core/IRuntime.cs

@@ -1,4 +1,3 @@
-using System;
 using LightInject;
 
 namespace Umbraco.Core

src/Umbraco.Core/IRuntimeState.cs

@@ -1,5 +1,6 @@
 using System;
 using Semver;
+using Umbraco.Core.Exceptions;
 using Umbraco.Core.Sync;
 
 namespace Umbraco.Core
@@ -55,5 +56,10 @@ namespace Umbraco.Core
         /// Gets the runtime level of execution.
         /// </summary>
         RuntimeLevel Level { get; }
+
+        /// <summary>
+        /// Gets the exception that caused the boot to fail.
+        /// </summary>
+        BootFailedException BootFailedException { get; }
     }
 }

src/Umbraco.Core/RuntimeState.cs

@@ -4,6 +4,7 @@ using System.Web;
 using Semver;
 using Umbraco.Core.Configuration;
 using Umbraco.Core.Configuration.UmbracoSettings;
+using Umbraco.Core.Exceptions;
 using Umbraco.Core.Logging;
 using Umbraco.Core.Sync;
 
@@ -87,7 +88,7 @@ namespace Umbraco.Core
         /// </summary>
         public RuntimeLevel Level
         {
-            get { return _level; }
+            get => _level;
             internal set { _level = value; if (value == RuntimeLevel.Run) _runLevel.Set(); }
         }
 
@@ -113,5 +114,10 @@ namespace Umbraco.Core
         {
             return _runLevel.WaitHandle.WaitOne(timeout);
         }
+
+        /// <summary>
+        /// Gets the exception that caused the boot to fail.
+        /// </summary>
+        public BootFailedException BootFailedException { get; internal set; }
     }
 }

src/Umbraco.Web/UmbracoModule.cs

@@ -2,6 +2,7 @@
 using System.Collections;
 using System.Collections.Generic;
 using System.IO;
+using System.Text;
 using System.Web;
 using System.Web.Routing;
 using LightInject;
@@ -504,7 +505,31 @@ namespace Umbraco.Web
             if (Core.Composing.Current.RuntimeState.Level == RuntimeLevel.BootFailed)
             {
                 // there's nothing we can do really
-                app.BeginRequest += (sender, args) => throw new BootFailedException("Boot failed. Umbraco cannot run. Umbraco's log file contains details about what caused the boot to fail.");
+                app.BeginRequest += (sender, args) =>
+                {
+                    // would love to avoid throwing, and instead display a customized Umbraco 500
+                    // page - however if we don't throw here, something else might go wrong, and
+                    // it's this later exception that would be reported. could not figure out how
+                    // to prevent it, either with httpContext.Response.End() or .ApplicationInstance
+                    // .CompleteRequest()
+
+                    // also, if something goes wrong with our DI setup, the logging subsystem may
+                    // not even kick in, so here we try to give as much detail as possible
+
+                    Exception e = Core.Composing.Current.RuntimeState.BootFailedException;
+                    if (e == null)
+                        throw new BootFailedException(BootFailedException.DefaultMessage);
+                    var m = new StringBuilder();
+                    m.Append(BootFailedException.DefaultMessage);
+                    while (e != null)
+                    {
+                        m.Append($"\n\n-> {e.GetType().FullName}: {e.Message}");
+                        if (string.IsNullOrWhiteSpace(e.StackTrace) == false)
+                            m.Append($"\n{e.StackTrace}");
+                        e = e.InnerException;
+                    }
+                    throw new BootFailedException(m.ToString());
+                };
                 return;
             }
 
@@ -548,14 +573,15 @@ namespace Umbraco.Web
 		    app.EndRequest += (sender, args) =>
 		    {
 		        var httpContext = ((HttpApplication) sender).Context;
+
 		        if (UmbracoContext.Current != null && UmbracoContext.Current.IsFrontEndUmbracoRequest)
 		        {
 		            Logger.Debug<UmbracoModule>($"End Request. ({DateTime.Now.Subtract(UmbracoContext.Current.ObjectCreated).TotalMilliseconds}ms)");
 		        }
 
-                    OnEndRequest(new UmbracoRequestEventArgs(UmbracoContext.Current, new HttpContextWrapper(httpContext)));
+                OnEndRequest(new UmbracoRequestEventArgs(UmbracoContext.Current, new HttpContextWrapper(httpContext)));
 
-                    DisposeHttpContextItems(httpContext);
+                DisposeHttpContextItems(httpContext);
 		    };
 		}