Return HTTP/400 when checkin.php input is invalid
Return HTTP/400 when checkin.php input is invalid

This page is generally used in automated context (web hooks, etc), so
just displaying the error in the HTML response is insufficient and not
easy for the caller to parse.

Returning an HTTP status code 400 (Bad Request) makes it clear that
a problem occurred.

Fixes #365

make-all/source-integration,mantisbt-plugins/source-integration,mantisbt-plugins/source-integration,mantisbt-plugins/source-integration,make-all/source-integration,make-all/source-integration