UI_process() didn't generate errors

Since there are many parts of UI_process() that can go wrong, it isn't
very helpful to only return -1 with no further explanation.  With this
change, the error message will at least show which part went wrong.

Reviewed-by: Rich Salz <rsalz@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/2037)