Make top-level errors consistent and observable

[glasser]

Previously, errors that weren't specially cased as part of the "request
pipeline" were not observable by plugins. (In some but not all cases,
they were visible to formatError hooks.) Some "HTTP-level" errors were
handled by returning text/plain responses, whereas many other errors
were handled by returning GraphQL-style JSON responses (using
the formatError hook).

This change makes things more consistent by returning all errors as JSON
responses (using the formatError hook). In addition, for better
observability, several new top-level plugin APIs are introduced that are
called when various kinds of errors occur. Specifically:

• New plugin hook startupDidFail. This is called once if startup fails
  (eg, if a serverWillStart plugin throws or if gateway fails to load
  the schema). (If this hook throws, its error is logged.) Future calls to
  executeHTTPGraphQLRequest (if the server was started in the
  background) now return a formatted JSON error instead of throwing.

• New plugin hook contextCreationDidFail. (If this hook throws, its
  error is logged.) As before, these errors are sent as formatted JSON
  errors.

• New plugin hook invalidRequestWasReceived. This is called for all
  the errors that occur before being able to construct a GraphQLRequest
  from an HTTPGraphQLRequest; this includes things like the top-level
  fields being of the wrong type, missing body, CSRF failure, etc).
  These errors are now sent as formatted JSON instead of as text/plain.

• New plugin hook unexpectedErrorProcessingRequest. This is called if
  processGraphQLRequest throws (typically because a plugin hook
  threw). We are making the assumption that this means there is a
  programming error, so we are making the behavior change that the
  thrown error will be logged and "Internal server error" will be sent
  in the HTTP response.

• Fix grammar of "GET supports only query operation" and don't return it
  if the operation is unknown.

Fixes #6140. Part of #6053.

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[glasser]

@trevor-scheer and I paired on this. I'm going to merge now though it would be nice if @trevor-scheer gave it another post-merge skim.

[trevor-scheer]

Approved in post, proposed followups in #6518