Fix crash when extras function doesn't return an array.

Fixes #745, which failed due to attempting to call `map` on a
non-array. This bails out at the same spot, but with a more
illuminating error message.

cli/src/generate/dsl.js

@@ -292,7 +292,12 @@ function grammar(baseGrammar, options) {
 
     extras = options.extras
       .call(ruleBuilder, ruleBuilder, baseGrammar.extras)
-      .map(normalize);
+
+    if (!Array.isArray(extras)) {
+      throw new Error("Grammar's 'extras' function must return an array.")
+    }
+
+    extras = extras.map(normalize);
   }
 
   let word = baseGrammar.word;