Improve the "Diffs didn't match" error message

[jen20]

Following up from a question in one of the public IRC rooms that @phinze and I were in, it seems there is scope to improve the "diff's don't match" error message with the additional information captured in the log to describe which attributes do not match.

Example: https://gist.github.com/nwalke/358d13e9e8520bc15276 (the relevant info is actually on the line preceeding the contents of this gist).

[phinze]

Done in #5276 👍

[ghost]

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