add EExpr error for unexpected comma

crates/reporting/src/error/parse.rs

@@ -687,6 +687,23 @@ fn to_expr_report<'a>(
                 severity: Severity::RuntimeError,
             }
         }
+        EExpr::UnexpectedComma(pos) => {
+            let surroundings = Region::new(start, *pos);
+            let region = LineColumnRegion::from_pos(lines.convert_pos(*pos));
+
+            let doc = alloc.stack([
+                alloc.reflow(r"I am trying to parse an expression, but I got stuck here:"),
+                alloc.region_with_subregion(lines.convert_region(surroundings), region),
+                alloc.concat([alloc.reflow("This comma in an invalid position.")]),
+            ]);
+
+            Report {
+                filename,
+                doc,
+                title: "UNEXPECTED COMMA".to_string(),
+                severity: Severity::RuntimeError,
+            }
+        }
         _ => todo!("unhandled parse error: {:?}", parse_problem),
     }
 }