Add didEncounterErrors to request pipeline API.

[abernix]

(As I mentioned at the bottom of #2714)

This adds a new life-cycle event to the new request pipeline API called didEncounterErrors, which receives requestContext's unformatted errors. (The requestContext represents an individual request within Apollo Server.) These errors give access to potentially different errors than those received within response.errors, which are sent to the client and available on the willSendResponse life-cycle hook, since they are not passed through the formatError transformation. This can allow plugin implementers to take advantage of the actual errors and properties which may be pruned from the error before transmission to the client.

While most other request pipeline life-cycle events provide a guarantee of their arguments, this didEncounterErrors will have a little bit less certainty since the errors could have happened in parsing, validation, or execution, and thus different contracts would have been made (e.g. we may not have been able to negotiate a requestContext.document if we could not parse the operation).

This should hopefully be a step in the right direction for desires like those expressed in #1709, and helps make that more of a reality when combined with the recent #2714.

This still needs tests and (Edit: Tests are now included in 705defb) I suspect I'll have some additional changes prior to this becoming official, but it can ship as-is for now and will live in the Apollo Server 2.6.0 alpha for the time-being. Use with minimal risk, but with the caveat that the final API may change. Also, I'll need to add docs to #2008 before we finally ship this.

[lorensr]

For anyone looking for more info on didEncounterErrors—it gets a GraphQLRequestContext:

https://github.com/apollographql/apollo-server/blob/master/packages/apollo-server-types/src/index.ts#L61-L92

which includes:

DocumentNode
https://github.com/graphql/graphql-js/blob/master/src/language/ast.js#L193-L198

OperationDefinitionNode
https://github.com/graphql/graphql-js/blob/master/src/language/ast.js#L209-L218

[abernix]

And just to clarify document is the DocumentNode, and operation is the OperationDefinitionNode, though neither is guaranteed since the error might have happened during parsing and validation, or might have occurred even earlier in the pipeline. This will be included in the documentation soon!