Improve error reporting

[str4d]

Things I've noticed:

• [ ] Error types should implement PartialEq (so they can be checked in tests).
  • Not doing any more (because plonk::Error now has a case that wraps std::io::Error which doesn't impl PartialEq).
• [x] Error types should implement std::error::Error. #338
• [ ] plonk::Error::NotEnoughRowsAvailable should indicate how many rows are needed (i.e. what k value to provide to MockProver::run or Params::new).
  • Update: I believe I already tried adding this, and it ended up not being useful because it would always indicate the "next power of two" because synthesis wasn't running in a way that could detect the actually-necessary size. So unless we come up with a more clever way to shortcut this, it probably isn't worth doing.
• [ ] plonk::Error::BoundsFailure does not distinguish between "too few rows to fit the desired circuit assignments within the usable rows" (which should be fixed by increasing the number of rows) and other bounds failures (due to programming error).
• [ ] plonk::Error::Synthesis carries no metadata with it, so it's impossible to determine from it where the synthesis error occurred.
• [ ] In some cases, plonk::Error::Synthesis is returned when a column is not equality-enabled (e.g. constants[1] in EccChip).

[0xSage]

Hi @str4d I'm thinking about picking up this task as a good first project - is it still relevant?

[str4d]

I think so! I improved the main error type in #338, but there is still more to do. I've updated the top post accordingly.