Better error message when passing wrong public key

[rugk]

When you pas the wrong public key (and e.g. accidentally use the signature) you get a base64 error.

# minisign -Vm minisign -P fsadf
base64 conversion failed

I think this error message could be improved as the user does not know what failed to be converted and generally you do not really know what you did wrong here.

[jedisct1]

a205a35 should improve this. Thanks!

[rugk]

Nice, but what about "valid" or "correct" rather than "actual"? Just an idea - I think this is more precise.

[jedisct1]

Because a signature that looks like a signature is not necessarily correct nor valid for the given message.