Make the error message of alarm-not-found clear

When creating a alarm combination with invalid project, ceilometer will
raise alarm-Not-Found error, even though the specified alarm ids are
existent. The error message is easily misleading.
This change gives a explicit presentation of this case.

Change-Id: Id189209ed4ccf27543f8dc628ca5f82eeeeeaa44
Closes-bug: #1320433
Merge "Make the error message of alarm-not-found clear"