IMAP FETCH behavior on not found messages

[remk]

When adding the support of James on the integration testing

of the gatling-imap project, we discovered some discrepancies between

the behavior of James and Cyrus on IMAP fetch. When doing a fetch with some non existing messages

Cyrus will do a best effort and return the existing messages whereas James will return a BAD response.

And in case of a fetch on an empty mailbox Cyrus will return a NO response where James will return a BAD one.

More details here :
linagora/gatling-imap#37

DOD

• make the behaviour of FETCH the same than cyrus.
  ie :
  - fetch with an empty response should return a NO response.
  - fetch with a partial response should return the existing messages
• add integration tests demonstrating those behaviours.

[chibenwa]

IMO it might affect some users on PROD, I would be in favour of including this to the current Sprint.