Previously, if a file or non-existent directory was added to
FileSystemWatcher, it would fail with the message “Folder must not be a
file”. While it suggests that the folder needs to be a directory, it
doesn’t make it clear that it also needs to exist. It also doesn’t
tell the user which folder caused the problem.

This commit updates the message to make it clear that the folder must
exist and must be a directory, and the include the name of the
problematic folder in the error message.

Closes gh-3918