Make exceptions thrown by promise.onError() suppress the original exception.

Author: ldaley

ratpack-core/src/main/java/ratpack/exec/internal/DefaultPromise.java

@@ -90,6 +90,7 @@ public void error(Throwable throwable) {
           errorHandler.execute(throwable);
           super.complete();
         } catch (Throwable e) {
+          e.addSuppressed(throwable);
           super.error(e);
         }
       }