Plugin API: Allow to short-circuit and fall back to `ExecutionResult`

I think maybe we can have an API that allow phases that happens before `execute` to do short-circuit and then other steps and being skipped and `execute` returns `ExecutionResult` with the custom error.

For example:
- `parse` do short-circuit with the parsing error
- `validate` skipped (0 errors)
- `contextFactory` skipped
- `rootValue` skipped
- `execute` is not being called and returns the parsing error from `parse` phase as `{ data: undefined, errors: [THE_ERROR] }`

Rough draft: https://github.com/dotansimha/envelop/pull/358

This should allow developers to intercept parse/validate errors using `useErorrHandler` and similar plugins, and also simplify some aspects of the request pipeline.