Improve error message if the function hashmap is not detected

[montyly]

https://github.com/crytic/echidna/blob/03787217d8a5af6bbc07f0ed36ca8a35436db898/lib/Echidna/Solidity.hs#L349-L361

If the user provides a wrong filterFunctions, echidna crashes and the error message is not really clear:

We should change Error processing function hashmaps to something like Please verify the "filterFunctions" option or something like that, and ideally shows the ones that not found

[gustavo-grieco]

Good catch, we should fix this

[gustavo-grieco]

This should be fixed in https://github.com/crytic/echidna/pull/1258

[gustavo-grieco]

Merged, so I'm closing this.