Change contract for ServiceMessage.errorType(), default value is -1 instead of exception

Author: segabriel

services-api/src/main/java/io/scalecube/services/api/ServiceMessage.java

@@ -171,9 +171,9 @@ public boolean isError() {
    * @return error type.
    */
   public int errorType() {
-    final String errorType = headers.get(ERROR_TYPE);
+    String errorType = headers.get(ERROR_TYPE);
     if (errorType == null) {
-      throw new IllegalStateException("Message is not an error");
+      return -1;
     }
     try {
       return Integer.parseInt(errorType);