watcher: Acquire lock early and give more friendly message

By opening the lock file early, other programs can lock the
state file to prevent ganeti-watcher from restarting daemons.
Using the pause feature is inherently prone to race conditions.

Before a traceback was logged when the lock file couldn't
be acquired. Now it'll be a more friendly message.

Signed-off-by: Michael Hanselmann <hansmi@google.com>
Reviewed-by: Guido Trotter <ultrotter@google.com>
Reviewed-by: Iustin Pop <iustin@google.com>