Improve validation error message when field names conflict

[robzhu]

When I submit a query with overlapping field names like so:

node {
echo(val: "test"),
echo(val: "foo")
}

I get the following validation error:
"Fields "echo" conflict because they have differing arguments."

This PR modifies the error message to suggest using aliases when this occurs:
"Fields "echo" conflict because they have differing arguments. Use aliases on the fields to fetch both if this was intentional."

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[coveralls]

Changes Unknown when pulling aa72089 on robzhu:master into * on graphql:master*.

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[yuzhi]

If the reason is an array, Use aliases on the fields to fetch both if this was intentional. is going to be printed a lot of times. Is that intentional?

[leebyron]

Agree with @yuzhi - can you take a look at some of the output messages?

I patched this in to test. For example https://github.com/graphql/graphql-js/blob/master/src/validation/__tests__/OverlappingFieldsCanBeMerged-test.js#L308 now produces:

Fields "field" conflict because subfields "deepField" conflict because subfields "x" conflict because a and b are different fields. Use aliases on the fields to fetch both if this was intentional.. Use aliases on the fields to fetch both if this was intentional.. Use aliases on the fields to fetch both if this was intentional.

[coveralls]

Changes Unknown when pulling 7d2f191 on robzhu:master into * on graphql:master*.

[leebyron]

Ace

[robzhu]

Thanks for the quick review and merge!