Improve unexpected error handling and add test cases for it

app/assets/javascripts/darkswarm/services/checkout.js.coffee

@@ -22,11 +22,14 @@ Darkswarm.factory 'Checkout', ($injector, CurrentOrder, ShippingMethods, StripeE
           if response.data.path
             Navigation.go response.data.path
           else
+            throw response unless response.data.errors || response.data.flash
+
             Loading.clear()
             @errors = response.data.errors
             RailsFlashLoader.loadFlash(response.data.flash)
         catch error
-          RailsFlashLoader.loadFlash("Unkown error occurred")
+          RailsFlashLoader.loadFlash(error: t("checkout.failed")) # inform the user about the unexpected error
+          throw error # generate a BugsnagJS alert
 
     # Rails wants our Spree::Address data to be provided with _attributes
     preprocess: ->

spec/javascripts/unit/darkswarm/services/checkout_spec.js.coffee

@@ -6,7 +6,9 @@ describe 'Checkout service', ->
   flash = null
   scope = null
   FlashLoaderMock =
-    loadFlash: (arg)->
+    loadFlash: (arg) ->
+  Loading =
+    clear: (arg)->
   paymentMethods = [{
       id: 99
       test: "foo"
@@ -48,6 +50,7 @@ describe 'Checkout service', ->
     module 'Darkswarm'
     module ($provide)->
       $provide.value "RailsFlashLoader", FlashLoaderMock
+      $provide.value "Loading", Loading
       $provide.value "currentOrder", orderData
       $provide.value "shippingMethods", shippingMethods
       $provide.value "paymentMethods", paymentMethods
@@ -123,6 +126,29 @@ describe 'Checkout service', ->
         $httpBackend.flush()
         expect(Checkout.errors).toEqual {error: "frogs"}
 
+      it "throws an exception and sends a flash message to the flash service when reponse doesnt contain errors nor a flash message", ->
+        spyOn(FlashLoaderMock, "loadFlash") # Stubbing out writes to window.location
+        $httpBackend.expectPUT("/checkout.json").respond 400, "broken response"
+        try
+          Checkout.submit()
+          $httpBackend.flush()
+        catch e
+          expect(e.data).toBe("broken response")
+
+        expect(FlashLoaderMock.loadFlash).toHaveBeenCalledWith({ error: t("checkout.failed") })
+
+      it "throws an exception and sends a flash message to the flash service when an exception is thrown while handling the error", ->
+        spyOn(FlashLoaderMock, "loadFlash") # Stubbing out writes to window.location
+        spyOn(Loading, "clear").and.callFake(-> throw "unexpected error")
+        $httpBackend.expectPUT("/checkout.json").respond 400, {flash: {error: "frogs"}}
+        try
+          Checkout.submit()
+          $httpBackend.flush()
+        catch e
+          expect(e).toBe("unexpected error")
+
+        expect(FlashLoaderMock.loadFlash).toHaveBeenCalledWith({ error: t("checkout.failed") })
+
     describe "when using the Stripe Connect gateway", ->
       beforeEach inject ($injector, StripeElements) ->
         Checkout.order.payment_method_id = 666