Fix error messages.

Author: davidlehn

lib/events.js

@@ -99,7 +99,7 @@ api.unhandledEventHandler = function({event}) {
 // throw with event details
 api.throwUnacceptableEventHandler = function({event}) {
   throw new JsonLdError(
-    event.message,
+    'Unacceptable event occurred.',
     'jsonld.UnacceptableEvent',
     {event});
 };
@@ -120,7 +120,7 @@ api.strictModeEventHandler = [
     // fail on all warnings
     if(event.level === 'warning') {
       throw new JsonLdError(
-        event.message,
+        'Strict mode violation occurred.',
         'jsonld.StrictModeViolationEvent',
         {event});
     }