Prevent error messages while detecting OS

[Shadow53]

Using tide on openSUSE Tumbleweed causes the following to be printed out when opening a new shell:

cat: /etc/lsb-release: No such file or directory
math: Expected at least 1 args, got 0

This pull request fixes this by short-circuiting _tide_detect_os_linux_cases and returning 1 if the given file does not exist.

Description

At the top of _tide_detect_os_linux_cases, a check was added to test if the given path is a file and, if not, return 1.

Note: I considered using test -e to allow for symbolic links, but decided against it because it also returns true for
directories, which would be invalid. AFAIK, /etc/os-release and /etc/lsb-release are not symlinks on any distros, if they exist, so using -f should be fine? test -L could also be used, but I don't think it checks if the target of the link is a file, and implementing a thorough check using readlink is only something the occurred to me while writing this pull request and seems unnecessary for the files being checked.

How Has This Been Tested

I tested the logic manually in the shell just to be sure, but it's a simple enough line of code that I think it can be verified as correct by looking at it.

• I have tested using Linux.
• I have tested using MacOS.

Checklist

I do not think there are any tests for this, since it occurs during installation. If I am wrong, I can go back and modify tests. Similarly, I do not think there is any documentation related to this.

• I have updated the documentation accordingly.
• I have updated the tests accordingly.

[IlanCosman]

Just made some small stylistic changes. Thanks for the PR @Shadow53 😄