[FIX] check for invalid charge ranges in search engine adapters (see #963)

OMSSA crashes without any detailed error message if the charge range given by the user is empty (i.e., min_charge > max_charge). This pull request adds a check to (OMSSA|XTandem|MyriMatch)Adapter to test for the problem and exit with a meaningful log message if the problem was detected.

Also includes corresponding tests in `src/tests/topp/SEARCHENGINES`.

Reported in #963, fixes #963