watcher: Acquire lock early and give more friendly message
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 <68b67885ff74281219b4a4be33cc20fd45078d7c@google.com>
Reviewed-by: Guido Trotter <f62ca4d5358bdaeef0e50e57f9f7e231de63727e@google.com>
Reviewed-by: Iustin Pop <acf83a5274f31a43af6b914614365999469c1a31@google.com>

mbakke/ganeti,bitemyapp/ganeti,yiannist/ganeti,leshchevds/ganeti,andir/ganeti,yiannist/ganeti,onponomarev/ganeti,ganeti/ganeti,dimara/ganeti,leshchevds/ganeti,andir/ganeti,andir/ganeti,dimara/ganeti,ganeti/ganeti,bitemyapp/ganeti,mbakke/ganeti,ganeti-github-testing/ganeti-test-1,onponomarev/ganeti,apyrgio/ganeti,yiannist/ganeti,ganeti/ganeti,apyrgio/snf-ganeti,apyrgio/ganeti,ganeti-github-testing/ganeti-test-1,mbakke/ganeti,leshchevds/ganeti,grnet/snf-ganeti,apyrgio/snf-ganeti,grnet/snf-ganeti