Configurable server-level error handler for request errors

[rcardin]

Currently, request error handling is inconsistent:

• Path/query parameter errors (PathParamError, QueryParamError) are automatically caught in Route.matches and hardcoded to 400 Bad Request responses
• Body decoding errors (DecodingError from req.as[A]) must be manually handled by users via Raise.recover in every route handler

This forces boilerplate in every route that decodes a request body:

POST(p"/users") { req =>
  Raise.recover {
    val user = req.as[User]
    Response.created(user)
  } {
    case e: DecodingError => Response.badRequest(e.message)
    case ConnectionError  => Response.internalServerError("...")
  }
}

Additionally, the hardcoded error response format cannot be customized: Many projects use standardized error formats (e.g., RFC 7807 Problem Details) or custom JSON error envelopes.

Proposal

Introduce a configurable server-level error handler that unifies handling of all request-level errors (PathParamError, QueryParamError, DecodingError) with a sensible default.

New types:

• RequestError — enum wrapping all three error types (Path, Query, Body cases)
• ErrorHandler — type alias RequestError => Response with a default (400 Bad Request)

API changes:

• ServerDef gains an onError builder method for setting the error handler
• Route handler functions gain Raise[DecodingError] context so req.as[A] works without manual wrapping
• Route.matches and Routes.handle delegate to the configurable error handler instead of hardcoding responses

After:

// DecodingError propagates to server error handler — no manual wrapping
POST(p"/users") { req =>
  val user = req.as[User]
  Response.created(user)
}

// Custom error format at server level
val server = YaesServer.route(routes*).onError {
  case RequestError.Body(e)  => Response.badRequest(s"""{"error":"${e.message}"}""")
  case RequestError.Path(e)  => Response.badRequest(s"""{"error":"${e.message}"}""")
  case RequestError.Query(e) => Response.badRequest(s"""{"error":"${e.message}"}""")
}

Per-route override still works via Raise.recover inside the handler when needed.