Handle correctly QuotaError in EC2 API

Without this patch, EC2 API returns UnknownError when quota is exceeded, see bug #853381

This patch fixes that by handling the exception. Now euca-run-instances returns useful information:

$ euca-run-instances --instance-count 10 --instance-type m1.8gb.2cpus.diskless ami-00000012
QuotaError: Quota exceeded: code=InstanceLimitExceeded

Change-Id: I1f7b381b34d2f538b7cc2d9582a40fd8bf88fbc3
Signed-off-by: Julien Danjou <julien@danjou.info>

nova/api/ec2/__init__.py

@@ -629,6 +629,10 @@ def __call__(self, req):
             LOG.debug(_('InvalidRequest raised: %s'), unicode(ex),
                      context=context)
             return ec2_error(req, request_id, type(ex).__name__, unicode(ex))
+        except exception.QuotaError as ex:
+            LOG.debug(_('QuotaError raised: %s'), unicode(ex),
+                      context=context)
+            return ec2_error(req, request_id, type(ex).__name__, unicode(ex))
         except exception.InvalidInstanceIDMalformed as ex:
             LOG.debug(_('ValidatorError raised: %s'), unicode(ex),
                      context=context)