Improve error message for pokladot failing to decode block

[stwiname]

Description

Currently when a block cannot be decoded the error message just says that it failed to index block without any indication how to solve the issue.

The error should be updated for substrate with the following message:

Failed to index block ${blockNum}. This is because the block cannot be decoded. To solve this you can either:
* Skip the block
* Update the chain types. You can test this by viewing the block with https://polkadot.js.org/apps/
For further information please read the docs: https://academy.subquery.network/