Implement source in error object

[eraclitux]

Some applications (es. Emberjs) use source object in json api error to show messages returned from backend to users.
This PR extends ErrorObject to optionally include source object.
ref:
https://stackoverflow.com/questions/31918565/handling-errors-with-the-now-default-ember-data-json-api-adapter
http://jsonapi.org/format/#error-objects

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[eraclitux]

I signed it!

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CLAs look good, thanks!

[eraclitux]

I just discovered #134 😥