Make failed PauseMonitor and UnpauseMonitor with no valid channel not close AMI session.

Make failed PauseMonitor and UnpauseMonitor with no valid channel not close AMI session.

I also went ahead and took a little time to make sure that the manager value
AMI_SUCCESS was used instead of just return 0 being thrown around everywhere since that's
how we handle this stuff these days.

(closes issue ASTERISK-19249)
Reporter: Jamuel Starkey
Patches:
	res_monitor.c-ASTERISK-19249.diff uploaded by Jamuel Starkey (license 5766)

git-svn-id: https://origsvn.digium.com/svn/asterisk/branches/1.8@352959 65c4cc65-6c06-0410-ace0-fbb531ad65f3