[krbdev.mit.edu #5745] NIM: FEATURE: KRB5: extended krb5 error support

[asanka@mit.edu via RT]

This patch modifies the NIM Kerberos v5 plug-in to use the
krb5_get_error_message() function to look up the error string if the
call to krb5_get_init_creds_password() fails.  If the call to
krb5_get_error_message() fails, the caller would failover to the
previous method of looking up a suitable error message based on the
error code.

Patch generated against /src/window/identity on the SVN trunk.

Depends on tickets #6045 and #6046.