deps: add google-cloud-error-reporting

somehow it was initializing itself on prod ok before this. how?

requirements.txt

@@ -33,6 +33,7 @@ google-cloud-appengine-logging==1.3.0
 google-cloud-audit-log==0.2.5
 google-cloud-core==2.3.2
 google-cloud-datastore==2.11.0
+google-cloud-error-reporting==1.9.1
 google-cloud-logging==3.5.0
 google-cloud-ndb==2.1.1
 google-cloud-tasks==2.13.1