Have func_curl log a warning when a curl request fails.

Review: https://reviewboard.asterisk.org/r/2403/
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Merged revisions 383460 from http://svn.asterisk.org/svn/asterisk/branches/1.8


git-svn-id: https://origsvn.digium.com/svn/asterisk/branches/11@383461 65c4cc65-6c06-0410-ace0-fbb531ad65f3