Add error handling for nonsensical sleeves and increases about pyknit HOT 3 CLOSED

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.)

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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.

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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!

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