PubSub: Don't throw exception to python interpreter.

[shileiyu]

Avoiding annoying traceback.

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[dhermes]

@shileiyu Thanks for sending this PR, could you expound a little bit on what problem this solves? I.e. could you provide a code snippet that causes the "annoying traceback" you refer to and then show the equivalent code snippet after this change?

[dhermes]

@shileiyu Thanks for taking the time to write and send this PR.

However, this issue is being resolve in a slightly different manner in #4472. Cheers!

[shileiyu]

@googlebot I signed it!