Convert return code to human readable string representing the error
code symbol itself. For example, gsasl_strerror_name(GSASL_OK)
returns the string "GSASL_OK".
This string can be used to output a diagnostic message to the user.
This function is one of few in the library that can be used without
a successful call to gsasl_init().
RETURN VALUE
Returns a pointer to a statically allocated string
containing a string version of the error code err, or NULL if
the error code is not known.