Log missing configuration files

[tkrajacic]

Once Environment providers allow for ignoring missing config files, they should probably log something when the file is not found.
It might be a user-error that the file isn’t there and this would be hard to detect otherwise.

[czechboy0]

Yeah currently only the ReloadingFileProvider depends on Swift Log (with a trait), so if we add logging to the stateless file-based providers, we need to do it in a way that maintains the same dependencies as today, and the logging only occurs when the LoggingSupport trait is explicitly enabled by the user.