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

test/cli/cli_test_case.rb

@@ -22,4 +22,8 @@ class CliTestCase < ActiveSupport::TestCase
     def stdouted
       capture(:stdout) { yield }.strip
     end
-end
+
+    def stderred
+      capture(:stderr) { yield }.strip
+    end
+  end