send better contexts to sis_pseudonym

fixes CANVAS-6YGK in sentry

test plan
 - specs should pass

Change-Id: If14754c714f67a67378663bc869191536fac4454
Reviewed-on: https://gerrit.instructure.com/189180
Tested-by: Jenkins
Reviewed-by: Cody Cutrer <cody@instructure.com>
QA-Review: Cody Cutrer <cody@instructure.com>
Product-Review: Cody Cutrer <cody@instructure.com>

app/models/message.rb

@@ -315,7 +315,7 @@ class Message < ActiveRecord::Base
       context = context.context if context.respond_to?(:context)
       context = context.account if context.respond_to?(:account)
       context = context.root_account if context.respond_to?(:root_account)
-      if context
+      if context && context.respond_to?(:root_account)
         p = SisPseudonym.for(user, context, type: :implicit, require_sis: false)
         context = p.account if p
       else

app/models/sis_pseudonym.rb

@@ -19,6 +19,7 @@ class SisPseudonym
   # type: :exact, :trusted, or :implicit
   def self.for(user, context, type: :exact, require_sis: true, include_deleted: false)
     raise ArgumentError("type must be :exact, :trusted, or :implicit") unless [:exact, :trusted, :implicit].include?(type)
+    raise ArgumentError("context must respond to .root_account") unless context.respond_to?(:root_account)
     self.new(user, context, type, require_sis, include_deleted).pseudonym
   end