Allow fallible HTTP handlers and map uncaught errors to 500 responses

[yorkin-bot]

Summary

HTTP handlers and middleware are currently required to be noraise:

• HttpHandler
• Middleware and MiddlewareNext
• the middleware execution chain
• dispatch_http is also noraise

This prevents ordinary fallible operations—JSON decoding, filesystem access, database calls, SSR, upstream HTTP calls—from propagating to a central error boundary.

Reproduction

This natural handler does not type-check:

app.post("/json", event => {
  let payload : Json = event.req.json()
  payload
})

moon check --target native reports:

Error [4122]: Function with error can only be used inside a function
with error types in its signature.

Applications must therefore catch every error inside every handler, discard it, or use an aborting construct such as try!. None of those choices allows consistent logging and conversion to an HTTP 500 response.

Expected behavior

Handlers and middleware should be allowed to raise checked errors. Mocket should catch unhandled request errors at one well-defined boundary and:

• return a default 500 response without terminating the process;
• offer an error hook/middleware for logging and custom responses;
• preserve cancellation rather than treating it as an ordinary request failure;
• keep successfully handled errors local when the application chooses to catch them.

Existing noraise handlers should remain valid inputs to the more general handler type.

[oboard]

fix: allow request handlers to raise errors- #25