Backend crashes on RedHat platforms when certain folders aren't accesible

[veggiesaurus]

The backend throws a std::filesystem_error exception and crashes when some folders aren't accessible on certain linux distributions (currently only reproducible on RHEL7) .

For example, if /etc/carta or ~/.carta isn't accessible, the backend can crash with an error in the log:

[critical] filesystem error: status: Permission denied.

I suspect it has to do with not catching exceptions thrown from std::filesystem::exists calls.

[veggiesaurus]

Actually, I can reproduce this on ubuntu as well.

For example, calling fs::exists("/root") as a normal user works fine (it returns true), but calling fs::exists("/root/somefile") results in an error getting thrown. This presents problems if the user can't access /etc/carta, or something has been messed up and the user is trying to access a different user's $HOME/.carta (:facepalm: :facepalm: :facepalm: this should never happen, but people don't always follow instructions with carta-controller)