Romloader: Add `DontTryOtherCores` config setting

If set, only the first preference core (whether it be through gamedb or preferredcore or priority or whatever) will be tried, and a failure for it will immediately fail the entire thing.  This is mostly a developer feature to aid in debugging.

src/BizHawk.Client.Common/RomLoader.cs

@@ -399,7 +399,7 @@ namespace BizHawk.Client.Common
 				}
 				catch (Exception e)
 				{
-					if (e is MissingFirmwareException || e.InnerException is MissingFirmwareException)
+					if (_config.DontTryOtherCores || e is MissingFirmwareException || e.InnerException is MissingFirmwareException)
 						throw;
 					exceptions.Add(e);
 				}

src/BizHawk.Client.Common/config/Config.cs

@@ -337,6 +337,8 @@ namespace BizHawk.Client.Common
 			["SGX"] = CoreNames.TurboNyma
 		};
 
+		public bool DontTryOtherCores { get; set; }
+
 		// ReSharper disable once UnusedMember.Global
 		public string LastWrittenFrom { get; set; } = VersionInfo.MainVersion;