Field errors

[gabrielf]

From the discussion of #44.
According to the spec on scalars under "Result Coercion":

If the server encounters some value that cannot be reasonably coerced 
to an Int, then it must raise a field error.

However it seems most examples and tests simply return null instead. What's correct?

[leebyron]

They raise a field error, but the error handling semantics (http://facebook.github.io/graphql/#sec-Error-handling) explain:

If an error occurs when resolving a field, it should be treated as though the field returned null, and an error must be added to the “errors” list in the response.

[leebyron]

Though that clause needs some explanation for non-nullable typed fields. I'll keep this open to handle that.

[leebyron]

graphql/graphql-spec@c589e2e