I ran into this ValueError while working on https://github.com/astral-sh/ruff/blob/main/scripts/fuzz-parser/fuzz.py, and found it slightly confusing at first. The issue we were running into was that a race condition (from running many fuzzing processes in parallel in a ProcessPoolExecutor) was meaning that our contains_bug() function was flaky -- so pysource-codegen would report that there was a bug, but then pysource-minimize wouldn't be able to repro the bug, causing the ValueError to be raised. But the exception message made me think that ruff's parser was erroring out because of some odd whitespace or other trivia that would be erased from the ast.parse()/ast.unparse() roundtrip, and that wasn't the case.

This PR attempts to improve the error message here slightly. It also adds a dedicated exception class, so that I can do except CouldNotMinimize here instead of except ValueError (which feels not great, since a lot of things could raise ValueError; it's hard to know that the ValueError is coming from pysource-minimize specifically there :-)