In UI_OpenSSL's open(), generate an error on unknown errno

TTY_get() sometimes surprises us with new errno values to determine if
we have a controling terminal or not.  This generated error is a
helpful tool to figure out that this was what happened and what the
unknown value is.

Reviewed-by: Rich Salz <rsalz@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/2043)