[krbdev.mit.edu #8631] Improve error message for GSS service ticket mismatch

[Greg Hudson via RT]

If you do "kinit -S servicename" and then contact that server principal 
using GSSAPI, it works, assuming you used the correct service name.

If you use the wrong service name, you get the error message "Matching 
credential not found" (and the ccache filename, after ticket 8052).  
This message is unclear, firstly because it talks about an 
implementation concern (finding a ticket that matches criteria chosen 
by krb5_get_credential()) and secondly because it doesn't provide 
enough parameters--most importantly for this use case, what service 
principal we were looking for.