framework(engine): 'Exit' should return 'TerminatedError' to trigger 'StopImpl'

[maxshuang]

What did you do?

The semantic of Exit was mistakenly changed by https://github.com/pingcap/tiflow/pull/6792. Since Exit should trigger StopImpl, so It need to return TerminateError.

What did you expect to see?

No response

What did you see instead?

.

Versions of the cluster

Dataflow Engine version (run tiflow version):

master