DEFAULT_ESPLORA_SERVER_URL is not useful for networks other than Network::Bitcoin

[rustaceanrob]

Calling Builder::set_network without changing the chain source will result in a Network::Bitcoin configuration for the chain source, using the mainnet Esplora server, but a test network wallet that will never sync. IMO there should be an error added to build error that reflects this network mismatch.

[tnull]

Calling Builder::set_network without changing the chain source will result in a Network::Bitcoin configuration for the chain source, using the mainnet Esplora server, but a test network wallet that will never sync. IMO there should be an error added to build error that reflects this network mismatch.

Hmm, there is no way to tell from the set URL for which network it is. We currently simply expect users to change the chain source to their liking whenever they also switch the network.

One solution might be to disallow setting the network independently from the chain source. If we do this, we probably should do it in conjuction with #117.