Improve advice for sync ops when db.lck is present

Improve advice for sync ops when db.lck is present

When the database is locked, sync operations involving transactions, such as
pacman -Syy, show the following:

:: Synchronizing package databases...
error: failed to update core (unable to lock database)
error: failed to update extra (unable to lock database)
error: failed to update community (unable to lock database)
error: failed to update multilib (unable to lock database)
error: failed to synchronize any databases

Whereas pacman -U <pkg> shows:

error: failed to init transaction (unable to lock database)
  if you're sure a package manager is not already
  running, you can remove /var/lib/pacman/db.lck

Which is much more meaningful, since the presence of db.lck may indicate an
erroneous lockfile instead of an ongoing transaction.

Improve the error messages for sync operations by advising the user to remove
db.lck if he is sure that no package manager is running.

Signed-off-by: Pang Yan Han <pangyanhan@gmail.com>
Signed-off-by: Dan McGee <dan@archlinux.org>