Tell the user if wings fails to mount a directory for a server

[DaniDipp]

I'm starting to experiment with mounts and thought that like docker mounts, the source folder would be created on my host machine but it just failed silently. I had to go to the wings logs to see that I have to manually create the source folder beforehand.

My suggestion is to add an error message to the panel that lets the user know that one of the mounts failed. I have no opinion on whether the server should fail to start if one of the mounts failed.