Improve check for nested writes
Merge pull request #2266 from tomchristie/improve-nested-write-checking

Improve error checking for nested writes.
Better errors when serializer has incorrectly named field.
Fix test for python2/3 compat
Merge pull request #2267 from tomchristie/better-misconfigured-serializer-errors

Better errors when serializer has incorrectly named field.