libnotmuch should catch QueryParserError seperately.

[David Bremner]

Today I observed the following

% notmuch search crash+date:@1505185200.. at 1521169200000
notmuch search: A Xapian exception occurred
A Xapian exception occurred parsing query: Unknown range operation
Query string was: crash+date:@1505185200.. at 1521169200000

This is roughly OK for a command line user, but contains very little
information for bindings users; I originally found this string
attempting to debug nmweb.

I _think_ it should be possible to catch the more specific error
here. Since this is an error that is generated by user input, it seems
worthwhile trying to do a better job.

d