gracefully handle inst-fs service errors

closes FOO-1327
flag=none

TEST PLAN:
  1) make inst-fs fail with 502
  2) upload something to canvas for SIS import
  3) user gets a 502

Change-Id: Id9c851b3246cd7b413f8a4428243b09d98477066
Reviewed-on: https://gerrit.instructure.com/c/canvas-lms/+/255072
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/sis_imports_api_controller.rb

@@ -364,6 +364,9 @@ class SisImportsApiController < ApplicationController
   include Api::V1::SisImport
   include Api::V1::Progress
 
+  rescue_from InstFS::ServiceError, with: :rescue_expected_error_type
+  rescue_from InstFS::BadRequestError, with: :rescue_expected_error_type
+
   def check_account
     return render json: {errors: ["SIS imports can only be executed on root accounts"]}, status: :bad_request unless @account.root_account?
     return render json: {errors: ["SIS imports are not enabled for this account"]}, status: :forbidden unless @account.allow_sis_import

lib/inst_fs.rb

@@ -520,8 +520,16 @@ module InstFS
   end
 
   class DirectUploadError < StandardError; end
-  class ServiceError < DirectUploadError; end
-  class BadRequestError < DirectUploadError; end
+  class ServiceError < DirectUploadError
+    def response_status
+      502
+    end
+  end
+  class BadRequestError < DirectUploadError
+    def response_status
+      400
+    end
+  end
   class ExportReferenceError < StandardError; end
   class DuplicationError < StandardError; end
   class DeletionError < StandardError; end