add extension.code field to BatchItemNotFoundError

[ryanruanwork]

Right now, dataloader-codegen will throw error when the number of items in request and response are not the same. see https://github.com/Yelp/dataloader-codegen/blob/master/src/implementation.ts#L249-L254

But given the nature of different implementation of resources(endpoints), developers using dataloader-codegen might want to catch this error in some cases.

For example, there is a resource named getUserMessage, it returns an empty array whenuserNotExist, this is an expected behaviour of the resource, but dataloader-codegen will throw a BatchItemNotFoundError. Developers may want to catch this error and return 0.

So, in this PR, I am going to export BatchItemNotFoundError error, so that developers will be able to catch this kind of error and return the default value they want.

[magicmark]

Are you sure we need to actually import apollo-server-express? As long as we can produce an error object that contains the code in the right place, we should be ok right?:

> const ApolloError = require('apollo-server-express').ApolloError;
undefined
> class FooError extends ApolloError{ constructor() { super('foo', 'FOO_BAR_BAZ_ERROR'); } }
undefined
> const e = new FooError('yikes')
undefined
> Object.keys(e)
[ 'extensions' ]
> Object.keys(e.extensions)
[ 'code' ]
> e.extensions.code
'FOO_BAR_BAZ_ERROR'

It looks like Apollo Server gets the error code by looking at .extensions.code - can we just add this to our error object?

also - could you provide a little more context in the PR description? What precisely are we trying to achieve here?

[magicmark]

Good stuff!