Improve UX around configuring Etherscan API key

[devdacian]

Describe the desired feature

Neither echidna --help nor the example config provide a way to set the etherscan api key. Apparently it can be set by setting an environment variable ETHERSCAN_API_KEY but ideally this would be mentioned somewhere in the --help output.

[ggrieco-tob]

https://github.com/crytic/building-secure-contracts/blob/master/program-analysis/echidna/advanced/state-network-forking.md#coverage-and-etherscan-integration

[rappie]

Making this available as a setting in the yaml config file would be great

[arcz]

We should also improve the error message to provide help right away

[elopez]

@arcz was there anything else you wanted to tackle in this issue other than the config file?