text: Title: Handle Exception on mangum
  Body: Thank you for creating great library!!
  
  I found the not good behavior when calling `exception_handler` with `Exception` on `FastAPI`
  
  I defined `exception_hanlder` for `Exception` which returns `JSONResponse`.
  ```
  @app.exception_handler(Exception)
  def all_exception_handler(_: Any, error: Exception):
      return JSONResponse(status_code=500, content={"message": error.args, "code": 500})
  ```
  
  I want to get a response on which status code is 500 with content.
  FastAPI(Starrett) raises `Exception`.
  Mangum doesn't handle the exception and lambda was die.  I can't get an excepted response from APIGateway.
  
  However,  `Uvicorn` handles the exception and returns expected response.
  
  Could you change Mangum to return excepted response?
  If you need the PR then, I can do it.
  
  Thank you.
  
  