feat(main): add PERMISSION_DENIED exit code

Signed-off-by: Christina Sørensen <christina@cafkafk.com>

src/main.rs

@@ -382,4 +382,7 @@ mod exits {
 
     /// Exit code for when the command-line options are invalid.
     pub const OPTIONS_ERROR: i32 = 3;
+
+    /// Exit code for when the you don't have permissions for the file.
+    pub const PERMISSION_DENIED: i32 = 13;
 }