Add tests for explicit context setting that check failure conditions and implement a helper function to generate a new breadcrumb stack.

src/renderer/walking.rs

@@ -42,6 +42,12 @@ where
     B: Borrow<dyn ContextElement + 'a>,
     P: Borrow<str>,
 {
+    if breadcrumbs.is_empty() {
+        // This happens when you use a section with an explicit
+        // context path, where both the path and the explicit context
+        // path fail, leaving you with absolutely no context.
+        return Err(WalkError::CantWalk);
+    }
     if path.is_empty() {
         return Ok(breadcrumbs
             .last()