Print ALL reasons a cached execution is not "skippable" about processcache HOT 1 CLOSED

I think a good way to implement this is as an enum of possible failure reasons:

enum CacheMissReason {
  BinaryMismatch,
  EnvVarMimsmatch,
  OutputFileChanged(PathBuf)
}

Notice you can even add the reason why it failed as "metadata" to the enum variant as data. Like in the OutputFileChanged example. We can give it the path of the offending file.

Then as you do your checks you can have a miss_reasons: Vec<CacheMissReasons> of why things fail thatt you append to:

miss_reasons.push_back(Foo)

And print them at the bottom by printing the Vec.

I know Joe didn't think it was super necessary, but I think it is clean, and later it if/when we add testing we can check the reasons why things fail match the expected reasons why things fail! The logic for checking for hash hits in nontrivial. So it would be nice to have tests for it. Specially since it will only get more complicated later.

from processcache.