Further improve malformed `.cue` UX

see 8f963ba58, #4013

src/BizHawk.Emulation.DiscSystem/DiscExtensions.cs

@@ -25,9 +25,10 @@ namespace BizHawk.Emulation.DiscSystem
 
 			if (discMountJob.OUT_ErrorLevel)
 			{
-				throw new Exception(string.IsNullOrEmpty(discMountJob.OUT_Log)
+				errorCallback(string.IsNullOrEmpty(discMountJob.OUT_Log)
 					? $"Could not process file \"{path}\"."
 					: $"Could not process file \"{path}\". Warnings/errors:\n{discMountJob.OUT_Log}\n(end disc load log)");
+				return discMountJob.OUT_Disc;
 			}
 
 			var disc = discMountJob.OUT_Disc;