improve error message reporting in romloader

fixes #2320

src/BizHawk.Client.Common/RomLoader.cs

@@ -677,23 +677,46 @@ namespace BizHawk.Client.Common
 			{
 				var system = game?.System;
 
+				DispatchErrorMessage(ex, system);
+				return false;
+			}
+
+			Rom = rom;
+			LoadedEmulator = nextEmulator;
+			Game = game;
+			return true;
+		}
+
+		private void DispatchErrorMessage(Exception ex, string system)
+		{
+			if (ex is AggregateException agg)
+			{
+				// all cores failed to load a game, so tell the user everything that went wrong and maybe they can fix it
+				if (agg.InnerExceptions.Count > 1)
+				{
+					DoLoadErrorCallback("Multiple cores failed to load the rom:", system);
+				}
+				foreach (Exception e in agg.InnerExceptions)
+				{
+					DispatchErrorMessage(e, system);
+				}
+
+				return;
+			}
+
 			// all of the specific exceptions we're trying to catch here aren't expected to have inner exceptions,
 			// so drill down in case we got a TargetInvocationException or something like that
 			while (ex.InnerException != null)
 				ex = ex.InnerException;
 
-				// Specific hack here, as we get more cores of the same system, this isn't scalable
 			if (ex is MissingFirmwareException)
 			{
-					DoLoadErrorCallback(ex.Message, system, path, Deterministic, LoadErrorType.MissingFirmware);
+				DoLoadErrorCallback(ex.Message, system, LoadErrorType.MissingFirmware);
 			}
 			else if (ex is CGBNotSupportedException)
 			{
 				// failed to load SGB bios or game does not support SGB mode.
-					// To avoid catch-22, disable SGB mode
-					_config.GbAsSgb = false;
-					DoMessageCallback("Failed to load a GB rom in SGB mode.  Disabling SGB Mode.");
-					return LoadRom(path, nextComm, launchLibretroCore, recursiveCount + 1);
+				DoLoadErrorCallback("Failed to load a GB rom in SGB mode.  You might try disabling SGB Mode.", system);
 			}
 			else if (ex is NoAvailableCoreException)
 			{
@@ -704,14 +727,6 @@ namespace BizHawk.Client.Common
 			{
 				DoLoadErrorCallback($"A core accepted the rom, but threw an exception while loading it:\n\n{ex}", system);
 			}
-
-				return false;
-			}
-
-			Rom = rom;
-			LoadedEmulator = nextEmulator;
-			Game = game;
-			return true;
 		}
 
 		/// <remarks>TODO add and handle <see cref="FilesystemFilter.LuaScripts"/> (you can drag-and-drop scripts and there are already non-rom things in this list, so why not?)</remarks>