Update web3py exception handling in proxies

[karlb]

Our proxies use the pattern

estimated_transaction = self.client.estimate_gas(...)
if estimated_transaction is not None:
    # error handling

But from what I recently observed, web3py does not return None in error cases (or at least not in all of them), but a ValueError. This requires changing the pattern to

try:
    estimated_transaction = self.client.estimate_gas(...)
except ValueError:
    # error handling

• Verify the changed behavior
• Devise a pattern to handle all relevant cases (other errors might cause other exceptions, or still return None)
• Implement pattern in all proxies

Alternatively, we could take advantage of ethereum/web3.py#941 and use the returned revert messages. This could make large parts of the error handling code unnecessary. But for that to work well, we need to add revert messages to the contracts first.

[karlb]

This error only occurs on master and is fixed by #6657