res_phoneprov: Cleanup module load error handling

Tested module load/reload interaction between res_phoneprov and
res_pjsip_phoneprov_provider in cases where res_phoneprov didn't
load correctly (usually misconfiguration or missing phoneprov.conf)

Tested-by: George Joseph

Review: https://reviewboard.asterisk.org/r/4069/
........

Merged revisions 425264 from http://svn.asterisk.org/svn/asterisk/branches/12


git-svn-id: http://svn.asterisk.org/svn/asterisk/branches/13@425265 f38db490-d61c-443f-a65b-d21fe96a405b