Bugfix/slack error handling

[mikeurbanski1]

This fixes two issues with how the Slack handler handles errors.

The first is a fix for the Exception message formatting, which resulted in a string concatenation error before, and the raised exception was actually this one, not the original exception. Now it uses string templates properly and raises the intended exception.

The second fix is to let any error with the UDF propagate up to the calling function, instead of returning None.

Before, catching this exception and returning None would result in alert_dispatcher thinking the handler completed successfully; it would set

result = {
  'success': True,
  'details': apply_some(handler_module.handle, **handler_kwargs),
}

where the return value of apply_some(...) was None.

This fix raises the exception up so that the result includes success: False and the exception that caused it.

[sfc-gh-afedorov]

perfect, thank you kindly