text: Title: FastAPI RegisterTortoise cannot add exception handlers during FastAPI lifespan
  Body: **Describe the bug**
  The RegisterTortoise class cannot add exception handlers during FastAPI lifespan
  
  **To Reproduce**
  Use this snippet:
  ```python
  from contextlib import asynccontextmanager
  
  import uvicorn
  from fastapi import FastAPI
  from tortoise import Model, fields
  from tortoise.contrib.fastapi import RegisterTortoise
  from tortoise.exceptions import DoesNotExist
  
  
  class Test(Model):
      id = fields.IntField(primary_key=True)
      name = fields.TextField()
  
  
  @asynccontextmanager
  async def lifespan(app: FastAPI):
      async with RegisterTortoise(app=app, db_url='sqlite://:memory:', modules={"models": ["__main__"]}, add_exception_handlers=True):
          yield
  
  app = FastAPI(lifespan=lifespan)
  
  
  @app.get("/")
  async def raise_error():
      raise DoesNotExist("Dummy")
  
  
  if __name__ == "__main__":
      uvicorn.run(app)
  ```
  
  Perform a GET on /:
  ```bash
  $ curl localhost:8000
  Internal Server Error
  ```
  
  **Expected behavior**
  Fix by explicitly adding exception handler before FastAPI init:
  ```python
  from contextlib import asynccontextmanager
  
  import uvicorn
  from fastapi import FastAPI, Request
  from fastapi.responses import JSONResponse
  from tortoise import Model, fields
  from tortoise.contrib.fastapi import RegisterTortoise
  from tortoise.exceptions import DoesNotExist
  
  
  class Test(Model):
      id = fields.IntField(primary_key=True)
      name = fields.TextField()
  
  
  @asynccontextmanager
  async def lifespan(app: FastAPI):
      async with RegisterTortoise(app=app, db_url='sqlite://:memory:', modules={"models": ["__main__"]}, add_exception_handlers=True):
          yield
  
  app = FastAPI(lifespan=lifespan)
  
  
  @app.get("/")
  async def raise_error():
      raise DoesNotExist("Dummy")
  
  
  @app.exception_handler(DoesNotExist)
  async def doesnotexist_exception_handler(request: Request, exc: DoesNotExist):
      return JSONResponse(status_code=404, content={"detail": "Not Found"})
  
  
  if __name__ == "__main__":
      uvicorn.run(app)
  ```
  
  Perform a GET on /, and receive a message handled by the exception handler.
  ```bash
  $ curl localhost:8000
  {"detail":"Not Found"}
  ```
  