[all] Quorum strangeness

[LayneHaber]

Describe the bug
Very strange error reported in #1275 -- working theory is:

1. JsonRpcProviders are created with a URL and do not specify a network. The provider will then allow for any network, and autodetect/update if there is a network change
2. A network change is detected (not sure why that is, but it's key to my theory)
3. There is no provider for that network, so ethers uses its FallbackProvider and resets the emitted blocks tag to -2 and tries to begin polling
4. All providers fail to get the block number (some seemed to be getting rate limited), and the provider throws the quorum error.

To test this theory (and hopefully narrow down if 3 is relevant or the root cause of it), we should specify networks when creating JsonRpcProviders to force them to throw instead of sync if a network change is detected.

To Reproduce
Not sure, have to ask XUD.