Set error message on KCM get_princ failure

This matches the expected behavior from other ccache types.  Most
notably, the KEYRING equivalent was added in
c25fc42e8eac7350209df61e4a7b9960d17755ca

(cherry picked from commit 58f60f3df7a625ccdcce23dfadd52dc335fd8da7)

https://github.com/krb5/krb5/commit/d48ad916d94c073aa9f208c2269e9da4a4a9a379
Author: Robbie Harwood <rharwood@redhat.com>
Committer: Greg Hudson <ghudson@mit.edu>
Commit: d48ad916d94c073aa9f208c2269e9da4a4a9a379
Branch: krb5-1.15
 src/lib/krb5/ccache/cc_kcm.c |    6 ++++++
 1 files changed, 6 insertions(+), 0 deletions(-)