"[ENHANCEMENT]" Do NOT drop batches that contain an invalid Splunk Index

[slcardinal]

What would you like to be added
If the index set by the SPLUNK_INDEX environment variable is invalid, do not drop the entire log batch. Instead of dropping the batch send logs with the invalid index to the lastchanceindex. This index should exist on all Splunk Cloud deployments.

Why is this needed:
To prevent data lose from a typo in the SPLUK_INDEX environment variable.

Additional context