ceph: only merge stderr on error

Previously we were merging the stderr even if it was empty, leading to
unmarshall errors.
The error simulation was done here
https://play.golang.org/p/Sk2yw9GUWNu.

Signed-off-by: Sébastien Han <seb@redhat.com>

pkg/daemon/ceph/client/command.go

@@ -164,7 +164,9 @@ func (c *CephToolCommand) run() ([]byte, error) {
 	if command == RBDTool {
 		if c.RemoteExecution {
 			output, stderr, err = c.context.RemoteExecutor.ExecCommandInContainerWithFullOutputWithTimeout(ProxyAppLabel, CommandProxyInitContainerName, c.clusterInfo.Namespace, append([]string{command}, args...)...)
-			output = fmt.Sprintf("%s. %s", output, stderr)
+			if stderr != "" || err != nil {
+				err = errors.Errorf("%s. %s", err.Error(), stderr)
+			}
 		} else if c.timeout == 0 {
 			output, err = c.context.Executor.ExecuteCommandWithOutput(command, args...)
 		} else {