Slim down validation failure message and use message type enum

The use of message type enum makes the git client colorize the
prefix (ERROR, WARNING or hint) and less text makes it easier to see
what's wrong, especially when the prefix is output for several commits
in a chain.

Old response when issue-id doesn't exist in tracker:

remote: Processing changes: refs: 1, updated: 1, done
remote: commit b37751b: Non-existing issue ids referenced in commit message
remote: The issue-ids
remote:     * TEST-123456
remote: are referenced in the commit message of
remote: b37751becd810a43f574524f4b01da10aead1705,
remote: but do not exist in its-jira Issue-Tracker

New (WARNING and hint are colorized):

remote: Processing changes: refs: 1, updated: 1, done
remote: WARNING: commit bc076d0: Non-existing issue-ids referenced in commit message
remote: hint: commit bc076d0: TEST-123456 not found in its-jira issue tracker

Change-Id: Iab9678f0174bf10007b6b536f15d01605c20d220