gh-126944: Better error for missing arg in pdb

[hauntsaninja]

• Issue: Better error message when missing args to pdb command #126944

[Wulian233]

Could we add more detailed error handling, even if it's just

expect Exception:

would be better

[gaogaotiantian]

This might be the minimum amount of code to fix the issue, but I don't think this is the correct place. It's not obvious why _getval would generate an error message. We should check the args for all the commands that require an argument (see many _print_invalid_args() calls in pdb.py) and print the error if the argument is missing.