Mina-signer: mention private key conversion function when `PrivateKey.fromBase58()` fails

[mitschabaude]

Several users of mina-signer have been trying to use it with out of range private keys and ran into an error.

That's why we introduced a conversion function to support these private keys. That function should be mentioned in the error message, whenever PrivateKey.fromBase58() fails with an error containing

Scalar: inputs larger than XXX are not allowed, got YYY

[emirsoyturk]

Hey I would like to help this issue if it is still available. @mitschabaude

[mitschabaude]

@emirsoyturk yes that would be great! It's available

[emirsoyturk]

Just opened a PR