BizHawkClient: Add error message if patching fails

[Zunawe]

What is this fixing or adding?

When a user opens a patch file and is asked to select their vanilla ROM through the settings api, if they close the file selection window or select a file that doesn't validate, the exception isn't visible to the user except through the console/log files. This adds a message to the client UI notifying the user what happened.

It's possible something like this should be closer to CommonClient or Settings, but would probably require quite a bit more UX design. At least compared to catching an error and logging something to the UI.

How was this tested?

Deleted my Emerald ROM and tried opening a patch file by providing

1. No file
2. An incorrect file
3. A correct file

If this makes graphical changes, please attach screenshots.