When CDRIVER-782 added error reporting for URI parsing, it updated _mongoc_uri_assign_read_prefs_mode() to report errors for unsupported read preference modes. For consistency, _mongoc_uri_build_write_concern() should also report an error if the "w" option is unsupported (as-is, it only logs a warning). This appears to be the lone case that was missed in CDRIVER-782, since conflicts between "journal" and "w" are caught by mongoc_write_concern_is_valid() in mongoc_uri_new_with_error().