Incorrect error messages returned from config parser #99

This is a regression during the refactor in ee0eb7c (#47). The nm_config_parse__line_* functions should have returned true on error, but were returning NULL (i.e. false) instead, causing nm_config_parse to treat it as an unknown line type instead of returning the more specific error message.

This regression does not cause any issues other than vague error messages being returned.

Possibly related to #98 (comment).