FIX: More error reporting improvements.

src/build/compile_test.rs

@@ -396,3 +396,25 @@ fn test_incomplete_tuple_key_value_missing_value_compile_failure() {
         ],
     )
 }
+
+#[test]
+fn test_format_expr_missing_expr_compile_error() {
+    assert_build_failure(
+        "\"foo\" %",
+        vec![
+            Regex::new(r"Expected format arguments").unwrap(),
+            Regex::new(r"at <eval> line: 1, column: 8").unwrap(),
+        ],
+    )
+}
+
+#[test]
+fn test_format_expr_unclosed_parens_compile_failure() {
+    assert_build_failure(
+        "\"foo\" % (1",
+        vec![
+            Regex::new(r"Expected \(\)\) but got \(\)").unwrap(),
+            Regex::new(r"at <eval> line: 1, column: 9").unwrap(),
+        ],
+    )
+}

src/parse/mod.rs

@@ -503,7 +503,8 @@ make_fn!(
     do_each!(
         tmpl => match_type!(STR),
         _ => punct!("%"),
-        args => either!(simple_format_args, expression_format_args),
+        args => wrap_err!(must!(either!(simple_format_args, expression_format_args)),
+            "Expected format arguments"),
         (Expression::Format(FormatDef {
             template: tmpl.fragment.to_string(),
             args: args,