feat(pubsub): expose common errors for easier handling

[hongalex]

Fixes #7925

[codyoss]

Are both of these errors we expect people to handle in their code or are they informational. I would say only expose these if you expect people to check for this error and take an action in code.

[codyoss]

nit: error message should start with a lower case letter. Also other errors in this package seem to prefix with pubsub. Should we do that here too?

[hongalex]

As is, this is to maintain complete backwards compatibility (the error message is completely the same in this case). We didn't standardize on prefixing with pubsub.

I also struggled with fixing this for ErrPublishingPaused, which starts in the middle of the previous error message to allow for error wrapping.

[hongalex]

Are both of these errors we expect people to handle in their code or are they informational. I would say only expose these if you expect people to check for this error and take an action in code.

Good question. For ErrTopicStopped, that topic is fully stopped and needs to be re-initialized before calling Publish again. Otherwise, the application can't continue. I suspect that this could actually be used the code (need to record that the Publish failed in this way)

For ErrTopicOrderingNotEnabled, this is probably more informational and wouldn't require any action to be taken in the code. I'll remove this.

[hongalex]

@codyoss @noahdietz I switched ErrPublishingPaused to a custom error that holds onto the ordering key. This leads to a slightly more elegant solution in that the string isn't awkwardly defined in two places, but now requires comparing with errors.As instead of errors.Is (since the ordering key can't be known beforehand). Thoughts on this approach instead?

[codyoss]

LGTM