include docker exeception cause in thread checker

SingularityExecutor/src/main/java/com/hubspot/singularity/executor/SingularityExecutorThreadChecker.java

@@ -123,7 +123,7 @@ private int getNumUsedThreads(SingularityExecutorTaskProcessCallable taskProcess
           dockerPid = Optional.of(possiblePid);
         }
       } catch (DockerException e) {
-        throw new ProcessFailedException(String.format("Could not get docker root pid due to error: %s", e));
+        throw new ProcessFailedException("Could not get docker root pid due to error", e);
       }
     }