Don't panic when cannot detect the arch package manager (fix #818)

src/error.rs

@@ -1,4 +1,5 @@
 use std::process::ExitStatus;
+
 use thiserror::Error;
 
 #[derive(Error, Debug, PartialEq)]
@@ -16,6 +17,10 @@ pub enum TopgradeError {
     #[error("Unknown Linux Distribution")]
     #[cfg(target_os = "linux")]
     UnknownLinuxDistribution,
+
+    #[error("Failed getting the system package manager")]
+    #[cfg(target_os = "linux")]
+    FailedGettingPackageManager,
 }
 
 #[derive(Error, Debug)]

src/steps/os/archlinux.rs

@@ -1,13 +1,16 @@
-use crate::execution_context::ExecutionContext;
-use crate::utils::which;
-use crate::{config, Step};
-use anyhow::Result;
 use std::env::var_os;
 use std::ffi::OsString;
 use std::path::{Path, PathBuf};
 use std::process::Command;
+
+use anyhow::Result;
 use walkdir::WalkDir;
 
+use crate::error::TopgradeError;
+use crate::execution_context::ExecutionContext;
+use crate::utils::which;
+use crate::{config, Step};
+
 fn get_execution_path() -> OsString {
     let mut path = OsString::from("/usr/bin:");
     path.push(var_os("PATH").unwrap());
@@ -171,7 +174,8 @@ pub fn get_arch_package_manager(ctx: &ExecutionContext) -> Option<Box<dyn ArchPa
 }
 
 pub fn upgrade_arch_linux(ctx: &ExecutionContext) -> Result<()> {
-    let package_manager = get_arch_package_manager(ctx).unwrap();
+    let package_manager =
+        get_arch_package_manager(ctx).ok_or_else(|| anyhow::Error::from(TopgradeError::FailedGettingPackageManager))?;
     package_manager.upgrade(ctx)
 }