Create deployment error (#4920)

Contribute towards b/440042223

It create a Deployment Error class and uses it for Deployment failure.

It also fixes the issuance of the metric for failures. It seems the way
that the metrics client is implemented, it's not allowed to emit the
metric in a exception handling block.

---------

Signed-off-by: Javan Lacerda <javanlacerda@google.com>

Author: javanlacerda

src/local/butler/deploy.py

@@ -51,6 +51,10 @@
 Version = namedtuple('Version', ['id', 'deploy_time', 'traffic_split'])
 
 
+class DeploymentError(Exception):
+  """Deployment Error"""
+
+
 def now(tz=None):
   """Used for mocks."""
   return datetime.datetime.now(tz)
@@ -445,7 +449,7 @@ def _prod_deployment_helper(config_dir,
   except Exception as ex:
     labels.update({'success': False})
     monitoring_metrics.PRODUCTION_DEPLOYMENT.increment(labels)
-    raise ex
+    raise DeploymentError from ex
 
 
 def _deploy_terraform(config_dir):