Introduce afs_error().

Introduce afs_error().

The callbacks of some afs commands employ the normal ->pbpout para
buffer to send an error message to the client on failure. These
messages are therefore tagged with the OUTPUT sideband designator
just as regular command output.

The receiving client writes such messages to stdout, so applications
which call para_client have no other way than parsing the output to
guess whether it is normal command output or an error message.

This commit improves on this by providing a public helper in afs.c
to format and send an error message that is tagged with the ERROR
sideband designator and thus gets written to stderr on the client
side. All afs callbacks which currently use ->pbout for error messages
are converted to call the new helper.