Invalid metrics expression returns 500

[drichelson]

When requesting an invalid metrics expression I would expect to get a 400 (Bad request) instead of the actual 500 (Internal Server Error) that is returned.
Example:

curl 'http://<host>/argusws/metrics?expression=INVALID_EXPRESSION' -v

{"status":500,"message":"Illegal metric expression found: [INVALID_EXPRESSION]"}

[asinghsfdc]

W-4985406 - added to 5b