sasl_usererr is called to convert a SASL return code (an integer) into a human
readable string. At this time the only language available is american
english written by programmers (aka gobbledygook).
Note that a server should call sasl_usererr on a return code first if
the string is going to be sent to the client.
saslerr specifies the error number to convert.
langlist is currently unused; Use NULL.
outlang specifies the desired RFC 1766 language for output. NULL defaults to "en-us,"
currently the only supported language.
It should be noted that this function is not the recommended means of
extracting error code information from SASL, instead application should
use sasl_errdetail(3), which contains this information (and more)