Improve parse errors in generated FromJSON instances

[danbroooks]

Fixes #1350

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[parsonsmatt]

this is great, thanks!

[jfraudeau]

Hello, I believe this PR introduced a breaking change:
When defining entities with FromJSON instances we now need to import Data.Aeson (withObject) while previously no Data.Aeson imports where needed.

withObject should probably be imported in persistent/Database/Persist/TH.hs

[parsonsmatt]

Great catch @jfraudeau ! opened #1369 to fix it