Prefix all on-chain error messages with contract/namespace

[krebernisak]

We should prefix all error messages with contract/namespace like so:

    with_attr error_message("Aggregator: Total amount due exceeds the balance"):

Originally posted by @krebernisak in #146 (comment)

[krebernisak]

We should also standardise the error language: OpenZeppelin/cairo-contracts#488