ReST API: Return 400 for malformed JSON input

Previously we had an unhandled exception that resulted in a 500 (Internal
Server Error) result and a stack trace when an invalid JSON file was
submitted in a POST/PUT request. Change to catching the exception and
returning a 400 (Bad Request) result.

bug 1102391

Change-Id: I7dde33ae732d9addbf988b1d36d4f06eacb634bf
Signed-off-by: Zane Bitter <zbitter@redhat.com>
Merge "ReST API: Return 400 for malformed JSON input"