Add validation of onError callback object for rules.

test/rules/any-blockquote.js

@@ -14,8 +14,7 @@ module.exports = {
       onError({
         "lineNumber": blockquote.lineNumber,
         "detail": "Blockquote spans " + lines + " line(s).",
-        "context": blockquote.line.substr(0, 7),
-        "range": null
+        "context": blockquote.line.substr(0, 7)
       });
     });
   }

test/rules/every-n-lines.js

@@ -13,9 +13,7 @@ module.exports = {
       if ((lineNumber % n) === 0) {
         onError({
           "lineNumber": lineNumber,
-          "detail": "Line number " + lineNumber,
-          "context": null,
-          "range": null
+          "detail": "Line number " + lineNumber
         });
       }
     });

test/rules/first-line.js

@@ -9,10 +9,7 @@ module.exports = {
   "function": function rule(params, onError) {
     // Unconditionally report an error for line 1
     onError({
-      "lineNumber": 1,
-      "detail": null,
-      "context": null,
-      "range": null
+      "lineNumber": 1
     });
   }
 };

test/rules/letters-E-X.js

@@ -17,9 +17,7 @@ module.exports = {
         if (index !== -1) {
           onError({
             "lineNumber": text.lineNumber,
-            "detail": null,
-            "context": text.content.substr(index - 1, 4),
-            "range": null
+            "context": text.content.substr(index - 1, 4)
           });
         }
       });