Add error code to parse/validate errors passed to 'didEncounterErrors'

[IvanGoncharov]

Implement the fix proposed in #6558 but implement it on top of AS4
Also, contain some cleanup and refactoring related to #6355

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[glasser]

Looks great. I like the nested error approach.

One thought -- would be good to have an end to end tests that shows how the errors are rendered. In ApolloServer.test.ts there's the test "parse errors" -- change it to do an inline snapshot on all of result, and add another test next to it for a validation error, perhaps?

[glasser]

(possibly as a follow-up) Worth thinking about if it's easy and a good idea to do something like #4351 (comment) ?

[glasser]

(though let's get this merged first before thinking of my last comment @IvanGoncharov )

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[IvanGoncharov]

@glasser I addressed you comment about adding more tests.
Sorry for delay.