API: Allow the Silences API to use their own 400 response

There's no change to the logic here - it just stroke me as odd that we were reusing the response from alerting groups in the silences call when it's pretty simple to add a new one in the API description.

This is more accurate as well, as this will be used to power the documentation.

Signed-off-by: gotjosh <josue.abreu@gmail.com>