Prevent crash for invalid LDML files

This fixes https://jira.sil.org/browse/WS-112 "Crash on Wesay
configuration Tool when importing .ldml file ...".
Is this the best (user-friendly) way to report the issue?  Is there a "standard way" to report back to the user that something didn't work as they expected?  MessageBox with the ex.Message doesn't seem the friendliest.