integrity: include keyring name for unknown key request

Depending on the IMA policy rule a key may be searched for in multiple
keyrings (e.g. .ima and .platform) and possibly not found.  This patch
improves feedback by including the keyring "description" (name) in the
error message.

Signed-off-by: Bruno Meneguele <bmeneg@redhat.com>
[zohar@linux.ibm.com: updated commit message]
Signed-off-by: Mimi Zohar <zohar@linux.ibm.com>