Improve error message for unbalanced brace. (#534)

Currently, an open brace without a corresponding brace will emit this confusing error message:

> Expected "!", "$", "&", "(", "*", "+", ".", "/", "/*", "//", ";", "?", character class, code block, comment, end of line, identifier, literal, or whitespace but "{" found.

This change adds an error case to the grammar to make it clear what the problem is.

src/parser.pegjs

@@ -437,6 +437,7 @@ AnyMatcher
 
 CodeBlock "code block"
   = "{" code:Code "}" { return code; }
+  / "{" { error("Unbalanced brace."); }
 
 Code
   = $((![{}] SourceCharacter)+ / "{" Code "}")*

test/unit/parser.spec.js

@@ -5,6 +5,9 @@ let parser = require("../../lib/parser");
 
 let expect = chai.expect;
 
+// better diagnostics for deep eq failure
+chai.config.truncateThreshold = 0;
+
 describe("PEG.js grammar parser", function() {
   let literalAbcd       = { type: "literal",      value: "abcd", ignoreCase: false };
   let literalEfgh       = { type: "literal",      value: "efgh", ignoreCase: false };
@@ -657,4 +660,16 @@ describe("PEG.js grammar parser", function() {
   it("parses EOF", function() {
     expect("start = 'abcd'\n").to.parseAs(trivialGrammar);
   });
+
+  it("reports unmatched brace", function() {
+    const text = "rule = \n 'x' { y \n z";
+    const errorLocation = {
+      start: { offset: 13, line: 2, column: 6 },
+      end: { offset: 14, line: 2, column: 7 }
+    };
+    expect(() => parser.parse(text))
+      .to.throw("Unbalanced brace.")
+      .with.property("location")
+      .that.deep.equals(errorLocation);
+  });
 });