python: Really throw an error if search_threads() fails

python: Really throw an error if search_threads() fails

In case, search_threads returns an error we are supposed to throw an
Exception. But we did not "raise" it, this was an oversight and this
commit fixes it.

There is still the problem that there is often output to stderr by
libnotmuch detailing the xapian error and this is simply printed
out. But this requires fixing at the libnotmuch level...

Signed-off-by: Sebastian Spaeth <Sebastian@SSpaeth.de>