Improve exception type and message

Author: trask

test/fakeIngestion/standalone/src/main/java/com/microsoft/applicationinsights/test/fakeingestion/MockedAppInsightsIngestionServer.java

@@ -113,7 +113,7 @@ public List<Envelope> waitForItems(final String type, final int numItems) throws
             }
         }, numItems, 10, TimeUnit.SECONDS);
         if (items.size() > numItems) {
-            throw new IllegalStateException("Received too many items of type " + type);
+            throw new AssertionError("Expecting " + numItems + " of type " + type + ", but received " + items.size());
         }
         return items;
     }