Emit a warning when the config file is found at the wrong location

[qexat]

The path for utoprc used to be $XDG_CONFIG_HOME/utoprc, but #484 has changed it to $XDG_CONFIG_HOME/utop/utoprc -- as the former was rightfully deemed incorrect. However, no measures for smooth transitioning from one to the other were taken ; as such, it is likely that many setups were affected by a breaking change with no notice on why code highlighting stopped working nor how to fix it.

While providing a fix for something adjacent in #506, I proposed to emit a warning for some time at the opening of utop if a file at the former location is detected.

[qexat]

I said I was going to work on it but after looking at the code for a bit I'm not really sure how to put that in practice as I'm not very familiar with utop's code base nor Lwt