Improved handling of trace queries / exceptions

[iamdefinitelyahuman]

What I did

Do not attempt to query a trace when the node client does not support it.

How I did it

1. Added the web3.supports_traces property method. This method calls debug_traceTransaction without any arguments and checks the returned error code. If the return code is -32601, the endpoint does not exist. Any other error means the endpoint does exist.
2. Within various TransactionReceipt methods, check for web3.supports_traces prior to attempting to query a trace. If False, give a more meaningful error than the standard "RPC endpoint does not exist / is not available".
3. Fix an issue where reverting transactions would raise if a trace is not available, because of an attempt to access the revert reason.

How to verify it

Run the tests. I've added some new cases.