Always return the same message when the draft workflow is disabled

Always return 'Draft workflow is disabled' as message when a draft
operation is rejected because the draft workflow is disabled by having
'change.allowDrafts' set to 'false'.

DeleteDraftChange and DeleteDraftPatchSet already returned this
message, but CreateChange and ReceiveCommits used another message.

Change-Id: I233e7ad1a2eadeab53f57b38a45b0cef8377d3b9
Signed-off-by: Edwin Kempin <edwin.kempin@sap.com>
6 files changed