Make PluginWithSuchNameExists more informative

To find out where are already imported modules of existing plugin
and the new one. We need to provide to the user places of allocations
of this modules.

Change-Id: Ic9e072dfb7f183c2d935b514d2b8df1bd42d3976

rally/common/plugin/plugin.py

@@ -13,6 +13,8 @@
 #    License for the specific language governing permissions and limitations
 #    under the License.
 
+import sys
+
 from rally.common.plugin import discover
 from rally.common.plugin import info
 from rally.common.plugin import meta
@@ -142,13 +144,17 @@ class Plugin(meta.MetaMixin, info.InfoMixin):
     @classmethod
     def _set_name_and_namespace(cls, name, namespace):
         try:
-            Plugin.get(name, namespace=namespace)
+            existing_plugin = Plugin.get(name, namespace=namespace)
         except exceptions.PluginNotFound:
             cls._meta_set("name", name)
             cls._meta_set("namespace", namespace)
         else:
-            raise exceptions.PluginWithSuchNameExists(name=name,
-                                                      namespace=namespace)
+            raise exceptions.PluginWithSuchNameExists(
+                name=name, namespace=namespace,
+                existing_path=(
+                    sys.modules[existing_plugin.__module__].__file__),
+                new_path=sys.modules[cls.__module__].__file__
+            )
 
     @classmethod
     def _set_deprecated(cls, reason, rally_version):

rally/exceptions.py

@@ -86,7 +86,9 @@ class PluginNotFound(NotFoundException):
 
 class PluginWithSuchNameExists(RallyException):
     msg_fmt = _("Plugin with such name: %(name)s already exists in "
-                "%(namespace)s namespace")
+                "%(namespace)s namespace. It's module allocates at "
+                "%(existing_path)s. You are trying to add plugin whose module "
+                "allocates at %(new_path)s.")
 
 
 class NoSuchConfigField(NotFoundException):