#403: Clarify error wording. (#409)

* #403: Clarify error wording.

* #403: Improve wording for payload vs options

sign.js

@@ -20,7 +20,7 @@ var sign_options_schema = {
   subject: { isValid: isString, message: '"subject" must be a string' },
   jwtid: { isValid: isString, message: '"jwtid" must be a string' },
   noTimestamp: { isValid: isBoolean, message: '"noTimestamp" must be a boolean' },
-  keyid: { isValid: isString, message: '"keyid" must be a string' },
+  keyid: { isValid: isString, message: '"keyid" must be a string' }
 };
 
 var registered_claims_schema = {
@@ -29,16 +29,16 @@ var registered_claims_schema = {
   nbf: { isValid: isNumber, message: '"nbf" should be a number of seconds' }
 };
 
-function validate(schema, unknown, object) {
+function validate(schema, allowUnknown, object, parameterName) {
   if (!isPlainObject(object)) {
-    throw new Error('Expected object');
+    throw new Error('Expected "' + parameterName + '" to be a plain object.');
   }
   Object.keys(object)
     .forEach(function(key) {
       var validator = schema[key];
       if (!validator) {
-        if (!unknown) {
-          throw new Error('"' + key + '" is not allowed');
+        if (!allowUnknown) {
+          throw new Error('"' + key + '" is not allowed in "' + parameterName + '"');
         }
         return;
       }
@@ -48,6 +48,14 @@ function validate(schema, unknown, object) {
     });
 }
 
+function validateOptions(options) {
+  return validate(sign_options_schema, false, options, 'options');
+}
+
+function validatePayload(payload) {
+  return validate(registered_claims_schema, true, payload, 'payload');
+}
+
 var options_to_payload = {
   'audience': 'aud',
   'issuer': 'iss',
@@ -97,7 +105,7 @@ module.exports = function (payload, secretOrPrivateKey, options, callback) {
     return failure(new Error('payload is required'));
   } else if (isObjectPayload) {
     try {
-      validate(registered_claims_schema, true, payload);
+      validatePayload(payload);
     }
     catch (error) {
       return failure(error);
@@ -122,7 +130,7 @@ module.exports = function (payload, secretOrPrivateKey, options, callback) {
   }
 
   try {
-    validate(sign_options_schema, false, options);
+    validateOptions(options);
   }
   catch (error) {
     return failure(error);