setusercontext(): Better error messages when priority is not set correctly

Polish the syslog messages to contain readily useful information.

Behavior of capability 'priority' is inconsistent with what is done for
all other contexts: 'umask', 'cpumask', resource limits, etc., where an
absence of capability means to inherit the value. It is currently
preserved for compatibility, but is subject to change on a future major
release.

Reviewed by: emaste, kib (older version)
Approved by: emaste (mentor)
MFC after: 3 days
Sponsored by: Kumacom SAS
Differential Revision: https://reviews.freebsd.org/D40349