Shrink generation failures

[MaximilianAlgehed]

When generation fails we simply give up and print a (usually enourmous) error message. However, what would be nice would be if we could shrink the variables we've already generated before the failing variable and backtrack, thus trying to find the smallest configuration that causes generation to fail.

[MaximilianAlgehed]

I realize now that doing this is not necessarily correct. One issue you get into is that earlier variables can shrink to things that couldn't be generated - thus giving you a weirdly inconsistent picture of why generation failed.