error: Use error_report_err() instead of ad hoc prints

error: Use error_report_err() instead of ad hoc prints

Unlike ad hoc prints, error_report_err() uses the error whole instead
of just its message obtained with error_get_pretty().  This avoids
suppressing its hint (see commit 50b7b00).  Example:

    $ bld/ivshmem-server -l 42@
    Parameter 'shm_size' expects a size
    You may use k, M, G or T suffixes for kilobytes, megabytes, gigabytes and terabytes.

The last line is new with this patch.

While there, drop a "cannot parse shm size: " message prefix; it's
redundant, because the error message proper is always of the form
"Parameter 'shm_size' expects ...".

Signed-off-by: Markus Armbruster <armbru@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Message-Id: <1450452927-8346-5-git-send-email-armbru@redhat.com>