Allow for graceful handling when port is already occupied

[nkitsaini]

I start uvicorn within same process as my application code using uvicorn.Server.serve(). If the port on which univcorn is supposed to run is already occupied, uvicorn exits the whole process.

The behavior makes sense when uvicorn is running as a standalon process but if not it would ideally be better if I can handle the error gracefully in my application code.

This can be a boolean parameter on config I think.