uexterr_format(): simplify output when ext error string is available

If the extended error string is provided by kernel, return only the
string, which is supposedly enough to identify exact cause of the error.
If the string is not provided, print the technically looking gibberish
which still allows to identify location with kernel sources.

Reviewed by: asomers, brooks
Sponsored by: The FreeBSD Foundation
Differential revision: https://reviews.freebsd.org/D51141