apps/app_voicemail: Demote an ERROR message to a WARNING message

When using IMAP voicemail with FreePBX, you will often get ERROR messages
complaining about not being able to find a mailbox. This is due to how FreePBX
handles voicemail mailboxes. Unfortunately, app_voicemail has to consider this
a configuration error, as in any other system it would be indicative of
someone misconfiguring their system.

Regardless, a misconfiguration is a WARNING, and not an ERROR. This patch
demotes the message so that system administrators can hopefully reduce some
of the noise in their log files.

Note that in the original patch this was made into a NOTICE, but that's a
too forgiving.

ASTERISK-24790 #close
Reported by: Graham Barnett
patches:
  app_voicemail.c.patch_noise uploaded by Graham Barnett (License 6685)
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Merged revisions 432098 from http://svn.asterisk.org/svn/asterisk/branches/11
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Merged revisions 432099 from http://svn.asterisk.org/svn/asterisk/branches/13


git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@432100 65c4cc65-6c06-0410-ace0-fbb531ad65f3