Describe the problem

When specifying wrong mountpoints composer errors with

ErrorEnqueueingJob, failed generating manifest: The following custom mountpoints are not supported [\"[SWAP]\"]" 

which results in a 500. It should be 400, the 500 sometimes results in alerts which shouldn't be alerts.

Steps to reproduce

1. Send an api request with wrong mountpoints to image builder.

Composer should error with some sort of identifiable error (separate error code etc…), so that image builder can make decisions based on it.