Rethrow cannot instantiate JQF runner via regular JUnit APIs

new IllegalArgumentException() did miss the cause exceptions, so
it was hard to debug the failure reason

fuzz/src/main/java/edu/berkeley/cs/jqf/fuzz/junit/GuidedFuzzing.java

@@ -168,10 +168,6 @@ public synchronized static Result run(Class<?> testClass, String testMethod,
         // Instantiate a runner (may return an error)
         Runner testRunner = testRequest.getRunner();
 
-        if (testRunner instanceof ErrorReportingRunner) {
-            throw new IllegalArgumentException(String.format("Could not instantiate a Junit runner for method %s#%s.", testClass.getName(), testMethod));
-        }
-
         // Start tracing for the test method
         SingleSnoop.startSnooping(testClass.getName() + "#" + testMethod);