requestPipeline: clean up some casts around error types

[glasser]

We now ensure various thrown things are Errors or GraphQLErrors as
appropriate.

[changeset-bot]

⚠️ No Changeset found

Latest commit: 7af859e

Merging this PR will not cause a version bump for any packages. If these changes should not result in a new version, you're good to go. If these changes should result in a version bump, you need to add a changeset.

This PR includes no changesets

When changesets are added to this PR, you'll see the packages that this PR includes changesets for and the associated semver types

Click here to learn what changesets are, and how to add one.

Click here if you're a maintainer who wants to add a changeset to this PR

[netlify]

❌ Deploy Preview for apollo-server-docs failed.

Name	Link
🔨 Latest commit	7af859e
🔍 Latest deploy log	https://app.netlify.com/sites/apollo-server-docs/deploys/62cf3bb58d27480009f73168

[codesandbox-ci]

This pull request is automatically built and testable in CodeSandbox.

To see build info of the built libraries, click here or the icon next to each commit SHA.

Latest deployment of this branch, based on commit 7af859e:

Sandbox	Source
Apollo Server Typescript	Configuration
Apollo Server	Configuration