Improve warning message if port needs admin permissions to bind

[levrik]

On Linux (and on WSL too) it requires admin permissions to run a server on a port under 1024.
Currently CRA just shows the message that there's already something running on that port if you try to do so without sudo or not as root.
I'm shifting my development stuff to WSL on Windows at the moment and didn't know that so it would be nice to show another message in this case.

For example:

? Admin permissions are needed to run a server on a port below 1024.
Would you like to run the app on another port instead? (Y/n)

ref #2512