Add additional information to the inconsistent cardinality error

[peterj]

At the moment the error message that's returned doesn't tell you much about the source of the error. It would be useful if the error included the name, label values and any other piece of data that could help identify the source.

[beorn7]

At some places, that might be more difficult than it looks, but I agree in principle.

[peterj]

I've sent out a PR that probably partially fixes this by outputting information that's available at that time.