Ignore DBus generated exceptions in USB scanning

These seem to happen on occasion for no particular reason, but
shouldn't kill the program.  See https://jira.sil.org/browse/WS-226
for an example of such a crash.  Several other issues involve such
exceptions, but each one seems to be a different exception.
Merge pull request #115 from sillsdev/Fix-WS-226

Ignore DBus generated exceptions in USB scanning