Fix for CONC-418:
Fix for CONC-418:

For unknown/not handled schannel error codes we now use the
FormatMessage function instead of returning "Unknown error" message.

ottok/mariadb-connector-c,ottok/mariadb-connector-c,MariaDB/mariadb-connector-c,MariaDB/mariadb-connector-c,MariaDB/mariadb-connector-c,ottok/mariadb-connector-c