FileNotFoundError when passing non existing key file to -k/--key

[DontPanicO]

Describe the bug
Exception FileNotFoundError not properly handled, when passing a non existing key file to -k/--key.
This happen only with asymmetric algorithms, since it's been handled with HS*

To Reproduce

1. Get a valid JWT (you can grab it from jwt.io)
2. Run an attack using an asymmetric algorithm and passing a non existing key file to -k

   jwtxpl <token> -a rs256 -p <key>:<value> -k /path/to/non/existing

Expected behavior
The script should properly handle this exception and return a custom error

[DontPanicO]

This issue is going to be closed with next release during the weekend