Handling errors when loading `chain.rlp`

[lightclient]

Right now, if nethermind (maybe other) clients fail to load chain.rlp because the block is invalid, they continue to startup. This can cause the test suite to run and then fail for non-obvious reasons. I think either i) clients should fail if chain.rlp fails to load or ii) hive should verify that the client has correctly loaded chain.rlp.

[fjl]

Continuing to run is the expected behavior in this case. In some simulators, we use chain.rlp and blocks/ for testing with invalid chains. Also, there is no good way for a client to tell hive why it failed to start. It is basically expected that clients will start for any combination of HIVE_ environment options and chain files.