Prevent AppDispatcher from interrupting app execution

Since every app is standalone, if one app fails to execute handler,
others should not be affected.

Change-Id: I674f7f306fc4868337c364312017d7845f83ff7c
Closes-Bug: #1622189

dragonflow/common/exceptions.py

@@ -10,6 +10,7 @@
 #    License for the specific language governing permissions and limitations
 #    under the License.
 
+from neutron_lib import exceptions
 from oslo_utils import excutils
 import six
 
@@ -88,3 +89,9 @@ class NoRemoteIPProxyException(DragonflowException):
 
 class LogicalPortNotFoundByTunnelKey(DragonflowException):
     message = _('Could not find logical port with tunnel key %(key)')
+
+
+class DFMultipleExceptions(exceptions.MultipleExceptions):
+
+    def __str__(self):
+        return ','.join(str(error) for error in self.inner_exceptions)