Print traceback to stderr if --debug is set

This change allows easier debugging of client related issues, with a
full traceback printed out when an uncaught error occurs in the client.
Previously, it could be hard to find the portion of code raising an
exception, with just the message from the exception being displayed.

Previous behaviour (with a rather artificial client error being raised):

    $ glance --debug image-list
    Contrived exception

Behaviour after this patch:

    $ glance --debug image-list
    Traceback (most recent call last):
      File "/opt/stack/python-glanceclient/glanceclient/shell.py", line 590, in main
        args.func(client, args)
      File "/opt/stack/python-glanceclient/glanceclient/v1/shell.py", line 77, in do_image_list
        raise(Exception('Contrived exception'))
    Exception: Contrived exception
    Contrived exception

Change-Id: Id7b76b707e5fc16a0402dcb104ec40787e0ffbe2

glanceclient/shell.py

@@ -26,6 +26,7 @@ import logging
 import os
 from os.path import expanduser
 import sys
+import traceback
 
 import six.moves.urllib.parse as urlparse
 
@@ -589,6 +590,12 @@ class OpenStackImagesShell(object):
             args.func(client, args)
         except exc.Unauthorized:
             raise exc.CommandError("Invalid OpenStack Identity credentials.")
+        except Exception:
+            #NOTE(kragniz) Print any exceptions raised to stderr if the --debug
+            # flag is set
+            if args.debug:
+                traceback.print_exc()
+            raise
         finally:
             if profile:
                 trace_id = osprofiler_profiler.get().get_base_id()