Improve signal processing error and log (#535)

* Improve signal processing error and log
* Remove closed check in stack trace.

Author: meiliang86

src/main/java/com/uber/cadence/internal/sync/DeterministicRunnerImpl.java

@@ -380,7 +380,6 @@ public String stackTrace() {
     StringBuilder result = new StringBuilder();
     lock.lock();
     try {
-      checkClosed();
       for (WorkflowThread coroutine : threads) {
         if (result.length() > 0) {
           result.append("\n");

src/main/java/com/uber/cadence/internal/sync/POJOWorkflowImplementationFactory.java

@@ -322,7 +322,10 @@ public void processSignal(String signalName, byte[] input, long eventId) {
         newInstance();
         signalMethod.invoke(workflow, args);
       } catch (IllegalAccessException e) {
-        throw new Error("Failure processing \"" + signalName + "\" at eventID " + eventId, e);
+        String errorMessage =
+            "Failed to process signal \"" + signalName + "\" at eventID " + eventId + ".";
+        log.error(errorMessage + "\n" + e);
+        throw new Error(errorMessage + " Check cause for details.", e);
       } catch (DataConverterException e) {
         logSerializationException(signalName, eventId, e);
       } catch (InvocationTargetException e) {
@@ -332,8 +335,10 @@ public void processSignal(String signalName, byte[] input, long eventId) {
         } else if (targetException instanceof Error) {
           throw (Error) targetException;
         } else {
-          throw new Error(
-              "Failure processing \"" + signalName + "\" at eventID " + eventId, targetException);
+          String errorMessage =
+              "Failed to process signal \"" + signalName + "\" at eventID " + eventId + ".";
+          log.error(errorMessage + "\n" + targetException);
+          throw new Error(errorMessage + " Check cause for details.", targetException);
         }
       }
     }