Error message for conflicting values could be nicer

[mikeando]

At the moment the message I get if I feed mismatched grids is:

ValueError: Unable to find unique value for key z_size, candidates: [23, 182]

To actually work out which variable was causing this I need to print out everything I can about each variable before it goes into the devito operator.

it would be much more helpful if the error message could say which variables those candidates were pulled from, something like:

ValueError: Unable to find unique value for key z_size, candidates: 
   182 - from u, v, vp
    23 - from savebuf