Rewrap errors in get_principal to CCacheError

Causes nicer error message when kerberos credentials are not available.

https://fedorahosted.org/freeipa/ticket/5272

Reviewed-By: David Kupka <dkupka@redhat.com>
Reviewed-By: Tomas Babej <tbabej@redhat.com>

install/tools/ipa-adtrust-install

@@ -310,7 +310,7 @@ def main():
 
     try:
         principal = krb_utils.get_principal()
-    except gssapi.exceptions.GSSError as e:
+    except errors.CCacheError as e:
         sys.exit("Must have Kerberos credentials to setup AD trusts on server: %s" % e.message)
 
     try: