API: fixes a few return codes for project snippets

When using project snippets via API the functions now provide status codes for
different situations other then only returning 404 error. If required parameters are missing,
e.g. `title` when creating a project snippet a 400 (Bad request) error is returned. The snippet
delete function now is idempotent and returns a 200 (Ok) regardless if the snippet with the
given id is available or not. Changing return codes of these functions has the advantage that
the 404 error is used only for resources, which are not available.

Tests added to check these status codes when handling project snippets.