tasks/cephfs: fix raising from RemoteProcess.wait()

...in the case where we call wait() on an already-finished
process.

Signed-off-by: John Spray <john.spray@redhat.com>

tasks/cephfs/vstart_runner.py

@@ -92,7 +92,10 @@ class LocalRemoteProcess(object):
         if self.finished:
             # Avoid calling communicate() on a dead process because it'll
             # give you stick about std* already being closed
-            return
+            if self.exitstatus != 0:
+                raise CommandFailedError(self.args, self.exitstatus)
+            else:
+                return
 
         out, err = self.subproc.communicate()
         self.stdout.write(out)