Runner should error out on invalid commands

[KerakTelor86]

This is really minor and probably not all that important so feel free to not waste your time on this.

So currently, the runner executable generated by tcframe runs in generation mode whenever an invalid command is issued e.g. ./runner grader. I think it probably should behave similarly to tcframe itself in these cases and list the available commands without running anything.

So I've had a few run-ins with this issue and my experience kind of looked like this:

• The output for generation and grading looks quite similar (in that there are test groups, test file names, and some form of good / bad feedback)
• The --solution argument (which our team usually uses for grading) works for generation too.
• Type in ./runner gr_s_de --solution=./clearly-wa-solution (underscores for emphasis)
• A bunch of OKs show up instead of the expected Wrong Answer.
• Re-run the command by tapping up arrow and pressing the return key. More OKs show up.
• Get really confused. Check clearly-wa-solution.cpp for mistakes. Check intended-solution.cpp for mistakes.
• Repeat for 15 minutes until either I realize I've made a typo or I realize it says Generation finished instead of Accepted at the bottom.

I realize this whole issue is really just my own fault, but I think it might save people a few minutes if it kind of errored out, y'know? I guess changing the message to Generation successful instead of OK would probably also work in this instance, although I'm not really sure which method would work better.

Thank you for reading, have a good day!