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?

In Palaso code (lib and apps), we almost never use message box. Instead, use Palaso.Reporting.NotifyUserOfProblem (or something like that). You can hand it the exception. Then you'll want a complete sentence... in this case something like "There was a problem reading the LDML (language/orthography definition file).". The dialog will give them a button they can use to report the full exception.