Better Error Handling for /v1/completion/typed

[r2d4]

Better error handling. Especially for TypeScript definition bugs. It's hard (impossible) to know how to fix something right now if it's broken (e.g., interface instead of type, missing syntax, etc.)

• Validation of the TypeScript Schema
• Syntax hints if invalid
• Possibly adding a compiler pre-check to the playground

[r2d4]

ref #1