fix: Properly exit process if function distro_version fails

`distro_version` contains code that can run `exit`, which is supposed to
exit the whole process. However, `distro_version` runs in a subshell.
By default in Bash, this means that `exit` will only exit the subshell, and set `?`
to that exit code. The rest of script will execute, despite the error.

There are two ways to fix this:
- Enable the POSIX shell option `errexit` for the entire script
- Manually check for non-zero exit code and "cascade the error"

The latter was chosen because it is a localized change that does not
affect the behavior of the entire script.

dkms.in

@@ -2325,7 +2325,7 @@ unset CC CXX CFLAGS CXXFLAGS LDFLAGS
 # Set important variables
 current_kernel=$(uname -r)
 current_os=$(uname -s)
-running_distribution=$(distro_version)
+running_distribution=$(distro_version) || exit
 dkms_tree="/var/lib/dkms"
 source_tree="/usr/src"
 install_tree="/lib/modules"