Improved deploy lock acquisition

1. Don't raise lock error for non-lock issues during lock acquire
  (see https://github.com/mrsked/mrsk/pull/181)
2. If there is an error while the lock is held, don't release the lock
  and send a warning to stderr

lib/mrsk/cli.rb

@@ -1,4 +1,5 @@
 module Mrsk::Cli
+  class LockError < StandardError; end
 end
 
 # SSHKit uses instance eval, so we need a global const for ergonomics