Improve "diffs didn't match" error output

[phinze]

Right now something called "diff one" and "diff two" is spit out when this error occurs, and with a bunch of manual editor munging you can usually work your way down to the central issue, but it seems that we could do a little more work in the error handling to make this less tedious.

[mitchellh]

Technically we can implement diff diffing. META.

[phinze]

Done in #1501

[ghost]

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