error: Don't append a newline when printing the error hint

error: Don't append a newline when printing the error hint

Since commit 50b7b00, we have error_append_hint() to conveniently
accumulate Error member @hint.  error_report_err() prints it with a
newline appended.  Consequently, users of error_append_hint() need to
know whether theirs is the final line of the hint to decide whether it
needs a newline.  Not a nice interface.

Change error_report_err() to print just the hint, and the (still few)
users of error_append_hint() to add the required newline.

Cc: Eric Blake <eblake@redhat.com>
Signed-off-by: Markus Armbruster <armbru@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Message-Id: <1450370121-5768-7-git-send-email-armbru@redhat.com>