feat: better error message on wrong os-release file (#511)

* enhancement: better error message when os-release parsing fails

src/error.rs

@@ -14,6 +14,10 @@ pub enum TopgradeError {
     #[cfg(target_os = "linux")]
     UnknownLinuxDistribution,
 
+    #[error("File \"/etc/os-release\" does not exist or is empty")]
+    #[cfg(target_os = "linux")]
+    EmptyOSReleaseFile,
+
     #[error("Failed getting the system package manager")]
     #[cfg(target_os = "linux")]
     FailedGettingPackageManager,

src/steps/os/linux.rs

@@ -114,10 +114,14 @@ impl Distribution {
         if PathBuf::from(OS_RELEASE_PATH).exists() {
             let os_release = Ini::load_from_file(OS_RELEASE_PATH)?;
 
+            if os_release.general_section().is_empty() {
+                return Err(TopgradeError::EmptyOSReleaseFile.into());
+            }
+
             return Self::parse_os_release(&os_release);
         }
 
-        Err(TopgradeError::UnknownLinuxDistribution.into())
+        Err(TopgradeError::EmptyOSReleaseFile.into())
     }
 
     pub fn upgrade(self, ctx: &ExecutionContext) -> Result<()> {