Use an extension method for fetching a required user in a common way (#15953)

src/Umbraco.Core/Extensions/UserServiceExtensions.cs

@@ -0,0 +1,11 @@
+using Umbraco.Cms.Core.Models.Membership;
+using Umbraco.Cms.Core.Services;
+
+namespace Umbraco.Cms.Core.Extensions;
+
+public static class UserServiceExtensions
+{
+    public static async Task<IUser> GetRequiredUserAsync(this IUserService userService, Guid key)
+        => await userService.GetAsync(key)
+           ?? throw new InvalidOperationException($"Could not find user with key: {key}");
+}