Give a more helpful error when a file is missing

Previously, this would just say "missing file". Now it shows the path of the file that was missing,
which should make it easier to debug what went wrong.

src/exercise.rs

@@ -201,14 +201,21 @@ path = "{}.rs""#,
     }
 
     pub fn state(&self) -> State {
-        let mut source_file =
-            File::open(&self.path).expect("We were unable to open the exercise file!");
+        let mut source_file = File::open(&self.path).unwrap_or_else(|e| {
+            panic!(
+                "We were unable to open the exercise file {}! {e}",
+                self.path.display()
+            )
+        });
 
         let source = {
             let mut s = String::new();
-            source_file
-                .read_to_string(&mut s)
-                .expect("We were unable to read the exercise file!");
+            source_file.read_to_string(&mut s).unwrap_or_else(|e| {
+                panic!(
+                    "We were unable to read the exercise file {}! {e}",
+                    self.path.display()
+                )
+            });
             s
         };