* GH-8770: Add `PostgresSubsChannel.errorHandler`
Fixes https://github.com/spring-projects/spring-integration/issues/8770
The problem with the `PostgresSubscribableChannel.notifyUpdate()` is that the try-catch block is outside the loop,
so the loop will die on an exception, leaving further messages unprocessed.
* Add ``PostgresSubscribableChannel.errorHandler` option to be invoked
after a `RetryTemplate` and for every failed message.
* The `askForMessage()` new logic is to catch an exception on a message and call `errorHandler`
returning a `FALLBACK_STUB` to continue an outer loop in the `notifyUpdate()`
**Cherry-pick to `6.1.x` & `6.0.x`**
* * Rename private `PostgresSubscribableChannel.askForMessage()` method to more specific `pollAndDispatchMessage()`
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