Pre: Make it easier to debug binding errors.

query-projector/src/lib.rs

@@ -164,7 +164,7 @@ fn candidate_column(query: &AlgebraicQuery, var: &Variable) -> (ColumnOrExpressi
     let columns = query.cc
                        .column_bindings
                        .get(var)
-                       .expect("Every variable has a binding");
+                       .expect(format!("Every variable should have a binding, but {:?} does not", var).as_str());
 
     let qa = columns[0].clone();
     let name = VariableColumn::Variable(var.clone()).column_name();
@@ -496,4 +496,4 @@ pub fn query_projection(query: &AlgebraicQuery) -> CombinedProjection {
             },
         }
     }
-}
+}