The two most common reasons that `mvt-ios decrypt-backup` can fail are
wrong passwords and not pointing to an actual backup.
We can distinguish these cases based on the kinds of errors thrown
from iOSbackup (at least for the current versions that i'm testing
with).
When we encounter those particular exceptions, just report a simple
summary and don't overwhelm the user with a backtrace. If we
encounter an unexpected exception, leave the reporting as-is.
Closes: #28, #36
