zebra: add label chunk range in log when label manager request fails

zebra: add label chunk range in log when label manager request fails

When the label manager is unable to provide a label chunk to
a routing service, an error message is displayed:

> Oct 11 11:47:27 vsr zebra[163745]: [YMY6E-K9JYD][EC 4043309085] Unable to assign Label Chunk to bgp instance 0

There is missing information on the range that was requested.
Add this information in the log message.

> Oct 11 11:47:27 vsr zebra[163745]: [YMY6E-K9JYD][EC 4043309085] Unable to assign Label Chunk 60 - 60 to bgp instance 0

Signed-off-by: Philippe Guibert <philippe.guibert@6wind.com>