Erroneous use of NTLM should be explicitely reported to the user

As seen in #2294 (closed) , misconfiguration of Windows clients are likely to lead to NTLM being attempted against LLNG when Kerberos auth is used.

The current error message might lead users to believe there is an issue with the server-side of things. We need a more explicit error message.

NTLM tickets are easy enough to spot:

00000000  4e 54 4c 4d 53 53 50 00  01 00 00 00 97 82 08 e2  |NTLMSSP.........|
00000010  00 00 00 00 00 00 00 00  00 00 00 00 00 00 00 00  |................|
00000020  0a 00 ba 47 00 00 00 0f                           |...G....|
00000028