Change php_verror so it doesn't double wrap varargs.

This means that zend compat extensions will get rational output from
php_verror.

This commit doesn't address the deeper issues of why php_verror is off
in the ghetto of ext/gd/libgd, nor why this wasn't caught with some kind
of testing.

This "fixes" https://github.com/facebook/hhvm/issues/4371