add error checking to jq and curl subshell invocation

[dekeonus]

• add err_msg function to divert output error stings to stderr
• add --fail to curl invocation to force it to exit non-zero on 404
• wrap jq in a function and add --exit-status to force non-zero exit on: parse failure || filter not found || filter result is null
• fix exit -> return in functions
• fix test for existing live ebuild (M.m.9999.ebuild) in revbump_blank.sh existing fix (full path via ${repo}) in zoneminder.sh left unchanged

[nabbi]

Thanks for improving this! #1