Add lazy error context for Rails error reporter (#2014)

Register a middleware with Rails.error that adds identity_id and
account_id from Current attributes. Only evaluated when an error
is actually reported, avoiding the cost on successful requests.

Gemfile.saas.lock

@@ -21,7 +21,7 @@ GIT
 
 GIT
   remote: https://github.com/basecamp/fizzy-saas
-  revision: 3b30f3d56d8318bb9256469cb91c06a5a5c760a6
+  revision: 43dbc896ce7b6a08194a92ddd1695d3f1ebf554b
   specs:
     fizzy-saas (0.1.0)
       audits1984

config/initializers/error_context.rb

@@ -0,0 +1,7 @@
+# Lazily add identity and account context to error reports.
+# Only evaluated when an error is actually reported.
+Rails.error.add_middleware ->(error, context:, **) do
+  context.merge \
+    identity_id: Current.identity&.id,
+    account_id: Current.account&.external_account_id
+end