Add error handling for nonsensical sleeves and increases

[Terri Oda]

I didn't put much in the way of error handling into the math functions, and we should probably fix that so people get a warning if they enter something that doesn't make sense.

Examples:

• The way increasing stitches with m1 (make 1) usually works, you can only add as many stitches as you had to start. So if increase_number > starting_count in the increase_evenly() function the instructions don't actually make sense. We should throw an error instead
• Entering 0s isn't going to make sense in a bunch of these either
• Negative numbers either. We could maybe import some sort of unsigned type, but for now I think just an error would be fine.

[Terri Oda]

Note: this is partially fixed by #34, which disallows negative numbers. We should still check for increase_number > starting_count and print an error.

(Yes, you could use double-increases, but I think it's rare that you'd want them in a sleeve because they tend to leave holes, so an error message so that folk double-check their numbers is likely more useful to the average user.)

[Ben Hampson]

I've added the errors that my non-knitting mind can think of. Let me know if there are some more to add.

I wonder if we should raise Exceptions instead? Currently a message is printed and the function carries on as normal. Also, if we had Exceptions we could also test for those scenarios.

[Terri Oda]

I filed an issue #38 about converting the errors to something that isn't a print statement. I'm probably not going to get to it until next week at the earliest, though!