Attempt to detect bad plan resolvers sooner

[benjie]

Description

Plan resolvers should return a step (or null); sometimes people forget the return statement and this can lead to misleading errors. Attempt to give a more sensible error up front.

Performance impact

Planning only.

Security impact

None known.

Checklist

• My code matches the project's code style and yarn lint:fix passes.
• I've added tests for the new feature, and yarn test passes.
• I have detailed the new feature in the relevant documentation.
• I have added this feature to 'Pending' in the RELEASE_NOTES.md file (if one exists).
• If this is a breaking change I've explained why.

[changeset-bot]

🦋 Changeset detected

Latest commit: f3f4592

The changes in this PR will be included in the next version bump.

This PR includes changesets to release 15 packages

Name	Type
graphile-utils	Patch
grafast	Patch
postgraphile	Patch
@localrepo/grafast-bench	Patch
@dataplan/json	Patch
@dataplan/pg	Patch
@grafserv/persisted	Patch
grafserv	Patch
ruru-components	Patch
@localrepo/grafast-website	Patch
graphile-build-pg	Patch
graphile-build	Patch
pgl	Patch
graphile-export	Patch
graphile	Patch

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