Add error handling for nonsensical gauge swatches.

[terriko]

If you make a gauge swatch with measurements set to 0, it's going to give you bad math. We should probably handle that with an appropriate error when you initially try to set something to 0 so you don't get surprised later.

We might also want to make some way for a user to specify "I never measured this part of my gauge" if they want to do measurement conversions for only rows or only stitches.