Add user-friendly error reporting for failed uploads

[tw4l]

Currently, METSFlask returns a 500 Internal Server Error if a user (1) tries to upload a file that is not a valid Archivematica METS file, or (2) the application runs into some other error during upload.

Add a user-friendly page at minimum for case 1, so that users know the issue is with the file they are uploading.