please provide informative error message for illegal MODHEX

[fschlich]

• YubiKey Manager (ykman) version: 4.0.0~a1
• How was it installed?: Debian package
• Operating system and version: Debian bullseye
• YubiKey model and version: YubiKey 5 Nano
• Bug description summary:

I wanted to program a yubiotp and supply my own public id, but didn't know what MODHEX is, so supplied an illegal value. The error message seems to point to a programming error. I would expect an error message that explains that the supplied value is illegal.

Steps to reproduce

ykman otp yubiotp --public-id vvvvvvvvvvvz --generate-private-id --generate-key 2
Error: substring not found

Expected result

ykman otp yubiotp --public-id vvvvvvvvvvvz --generate-private-id --generate-key 2
Error: not a valid MODHEX value (valid chars are c b d e f g h i j k l n r t u v): vvvvvvvvvvvz