openssl: fall back on SSL_ERROR_* string when no error detail

- If SSL_get_error is called but no extended error detail is available
  then show that SSL_ERROR_* as a string.

Prior to this change there was some inconsistency in that case: the
SSL_ERROR_* code may or may not have been shown, or may have been shown
as unknown even if it was known.

Ref: https://github.com/curl/curl/issues/1300

Closes https://github.com/curl/curl/pull/1348