feature request: Add revert reason to FailedRelayedMessage event

[transmissions11]

Is your feature request related to a problem? Please describe. It's a real pain to debug L1->L2 messages that revert when they're relayed. This is because they just show a generic "evm execution reverted" error.

Example: https://kovan-optimistic.etherscan.io/tx/0x647991fe8282b4ddd1fdb8dd201c412adc6cbb92eba021f47be182a8a0dfe00e/advanced

Describe the solution you'd like Adding the error message into FailedRelayedMessage would allow block explorers or the watcher to alert users and devs of why the relay failed, making it easier to debug.

Describe alternatives you've considered Doing a trace and looking directly for the revert in l2geth and exposing this to etherscan?

[smartcontracts]

Won't make it into 0.5.0 but this is a great candidate for 1.0.0.

[protolambda]

In bedrock the L1 to L2 messages have their own deposit txs, and the relay / entry-point on L2 is customizable. (there will likely be an encouraged default though that can replay failed deposit execution, and there this feedback is still useful). cc @maurelian