Document the new exception hierarchy

[fselmo]

In v6, we are introducing Web3Exception and inheriting eth_utils.exceptions.ValidationError across Web3ValidationError, EthPMValidationError and ENSValidationError. We should document this hierarchy and give examples of why this is useful / how to fine tune exception handling moving forward.

i.e.

try:
    ...

# catch only validation errors within web3
except Web3ValidationError:
    ...

# catch all web3 errors, including web3 validation errors
except Web3Exception:
    ...

# catch all validation errors across the web3 library stack (still needs to be reflected in `eth-tester`, `eth-abi`, `eth-account`, etc)
except ValidationError:
    ...

Changes related to standardizing the ValidationError will have to be coordinated for other libraries in the web3 stack:

• py-evm See: Unify ValidationError py-evm#1194
• eth-account
• eth-abi
• eth-tester
  ... ?

[kclowes]

Closed via #2778