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)
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Merged revisions 352959 from http://svn.asterisk.org/svn/asterisk/branches/1.8
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Merged revisions 352965 from http://svn.asterisk.org/svn/asterisk/branches/10


git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@352981 65c4cc65-6c06-0410-ace0-fbb531ad65f3