Prevent crash for invalid LDML files

This fixes https://jira.sil.org/browse/WS-112 "Crash on Wesay
configuration Tool when importing .ldml file ...".
I agree. MS coding guidelines say that catching Exception is a no-no. That should be especially true in a shared library and doing a quite large complex task.
I'd also think that for many kinds of exceptions we could give usefully more information, e.g., disk-full, syntax or other errors in the file (or obviously not a LIFT file at all). Conversely the program SHOULD crash if we get, say, a null reference error or divide by zero or similar things indicating programming mistakes.