Expose current executor's location

`Executor` has a `push_error` method which is very useful for reporting
back errors. However, this method needs to know the location of the
error (`SourcePosition`). This commit adds a `location` method to the
executor.

Author: rushmorem

juniper/src/executor.rs

@@ -254,6 +254,11 @@ impl<'a, CtxT> Executor<'a, CtxT> {
         self.fragments.get(name).map(|f| *f)
     }
 
+    /// The current location of the executor
+    pub fn location(&self) -> &SourcePosition {
+        self.field_path.location()
+    }
+
     /// Add an error to the execution engine
     pub fn push_error(&self, error: String, location: SourcePosition) {
         let mut path = Vec::new();