Add WhitehallError base class

As introduced in draft PR #11431. Sometimes we just want a generic
Whitehall error, or a way of catching multiple Whitehall errors,
without accidentally swallowing unexpected errors like NoMethodError
etc.

Author: ChrisBAshton

app/models/whitehall_error.rb

@@ -0,0 +1,14 @@
+# Base class for Whitehall-specific domain errors.
+#
+# We rescue WhitehallError in places where we want to handle
+# expected business/application failures gracefully, without also
+# swallowing unexpected framework or programming errors.
+#
+# Avoid rescuing StandardError directly unless we genuinely intend
+# to catch any application exception, including Rails, ActiveRecord,
+# or Ruby runtime errors.
+#
+# Any error inheriting from WhitehallError is considered part of
+# Whitehall's intentional public error surface.
+class WhitehallError < StandardError
+end