handle canvadoc 500s too

closes FOO-1317
flag=none

TEST PLAN:
  1) make canvadocs returen 500s
  2) treat as service unavailable
  3) no sentry report

Change-Id: Id3040aaf131d6af7707a3556e3b705b50ca9707d
Reviewed-on: https://gerrit.instructure.com/c/canvas-lms/+/254840
Tested-by: Service Cloud Jenkins <svc.cloudjenkins@instructure.com>
Reviewed-by: Charley Kline <ckline@instructure.com>
Product-Review: Charley Kline <ckline@instructure.com>
QA-Review: Ethan Vizitei <evizitei@instructure.com>

app/controllers/canvadoc_sessions_controller.rb

@@ -87,12 +87,15 @@ class CanvadocSessionsController < ApplicationController
   rescue HmacHelper::Error => e
     Canvas::Errors.capture_exception(:canvadocs, e, :info)
     render :plain => 'unauthorized', :status => :unauthorized
-  rescue Timeout::Error, Canvadocs::BadGateway => e
+  rescue Timeout::Error, Canvadocs::BadGateway, Canvadocs::ServerError => e
     Canvas::Errors.capture_exception(:canvadocs, e, :warn)
     render :plain => "Service is currently unavailable. Try again later.",
            :status => :service_unavailable
   rescue Canvadocs::BadRequest => e
     Canvas::Errors.capture_exception(:canvadocs, e, :info)
     render :plain => 'Canvadocs Bad Request', :status => :bad_request
+  rescue Canvadocs::HttpError => e
+    Canvas::Errors.capture_exception(:canvadocs, e, :error)
+    render :plain => 'Unknown Canvadocs Error', :status => :service_unavailable
   end
 end

lib/canvadocs.rb

@@ -149,6 +149,7 @@ module Canvadocs
       unless response.code =~ /\A20./
         err_message = "HTTP Error #{response.code}: #{response.body}"
         klass = Canvadocs::HttpError
+        klass = Canvadocs::ServerError if response.code.to_s == "500" 
         klass = Canvadocs::BadGateway if response.code.to_s == "502"
         klass = Canvadocs::BadRequest if response.code.to_s == "400"
         raise klass, err_message
@@ -199,6 +200,7 @@ module Canvadocs
 
   class Error < StandardError; end
   class HttpError < Error; end
+  class ServerError < HttpError; end
   class BadGateway < HttpError; end
   class BadRequest < HttpError; end