Improve compute-witness debugging info

[iAmMichaelConnor]

Currently, when passing an invalid set of arguments to compute-witness, the error returned doesn't help a human understand where in a .code file they should look, to understand which of their arguments could be causing the problem. It would be helpful to point a user to the 'human-readable' equality check (==) which has failed, and the exact line of the .code file they need to fix.

Would it be possible to retain a mapping from the R1CS constraints back to the original human-readable equality checks == from which they were derived?

[Schaeff]

Sure, makes total sense! I think #396 covers it?