feat: better error messages

Provide "No such file or directory" error if file is not found.
Provide "Unknown Error" if other error found

Should reduce confusion from the generic other error

tests/test_flags.rs

@@ -92,7 +92,7 @@ pub fn test_with_bad_param() {
     let mut cmd = Command::cargo_bin("dust").unwrap();
     let result = cmd.arg("bad_place").unwrap();
     let stderr = str::from_utf8(&result.stderr).unwrap();
-    assert!(stderr.contains("Did not have permissions for all directories"));
+    assert!(stderr.contains("No such file or directory"));
 }
 
 #[test]