chore: add descriptive error message to removeUnreachable

[tuner]

Nobody should see this message, but if they see, it probably makes a bit more sense after this PR.

Before:

TypeError: Cannot read properties of undefined (reading '$ref')
    at addReachable (/Users/klavikka/src/ts-json-schema-generator/src/Utils/removeUnreachable.ts:14:20)

After:

Error: Encountered a reference to a missing definition: "#/definitions/Union". This is a bug.
    at addReachable (/Users/klavikka/src/ts-json-schema-generator/src/Utils/removeUnreachable.ts:23:19)

[domoritz]

Thank you!

[github-actions]

🚀 PR was released in v0.97.1-next.0 🚀