[rqd] Change exitstatus for failed frames on docker that should be retried (#1692)

If a frame fails to be initialized because its container is not
available, mark the job with a statuscode that will get it back to
waiting to be retried automatically.
```
EXITSTATUS_FOR_FAILED_LAUNCH = 256
```

Author: DiegoTavares

rqd/rqd/rqcore.py

@@ -1195,12 +1195,12 @@ def runDocker(self):
                 self.rqlog.write(msg, prependTimestamp=rqd.rqconstants.RQD_PREPEND_TIMESTAMP)
         except InvalidFrameOsError as e:
             # Frame container didn't get created
-            returncode = -1
+            returncode = rqd.rqconstants.EXITSTATUS_FOR_FAILED_LAUNCH
             self.__writeHeader()
             self.rqlog.write(str(e), prependTimestamp=rqd.rqconstants.RQD_PREPEND_TIMESTAMP)
         # pylint: disable=broad-except
         except Exception as e:
-            returncode = -1
+            returncode = rqd.rqconstants.EXITSTATUS_FOR_FAILED_LAUNCH
             msg = "Failed to launch frame container"
             logging.exception(msg)
             self.rqlog.write("%s - %s" % (msg, e),