Re-throw stylesheet compile errors

This makes it consistent with other read errors. Previously a Notification
was returned in the error case causing errors downstream where the package's
stylesheets array was assumed to be a path/content tuple.

Closes atom/deprecation-cop#22

spec/fixtures/packages/package-with-invalid-styles/package.json

@@ -0,0 +1,4 @@
+{
+  "name": "package-with-invalid-styles",
+  "version": "1.0.0"
+}

spec/fixtures/packages/package-with-invalid-styles/styles/index.less

@@ -0,0 +1 @@
+{

spec/package-manager-spec.coffee

@@ -23,6 +23,12 @@ describe "PackageManager", ->
       expect(pack instanceof Package).toBe true
       expect(pack.metadata.name).toBe "package-with-broken-keymap"
 
+    it "returns the package if it has an invalid stylesheet", ->
+      pack = atom.packages.loadPackage("package-with-invalid-styles")
+      expect(pack instanceof Package).toBe true
+      expect(pack.metadata.name).toBe "package-with-invalid-styles"
+      expect(pack.stylesheets.length).toBe 0
+
     it "returns null if the package has an invalid package.json", ->
       spyOn(console, 'warn')
       expect(atom.packages.loadPackage("package-with-broken-package-json")).toBeNull()

spec/theme-manager-spec.coffee

@@ -364,12 +364,13 @@ describe "ThemeManager", ->
           throw new Error('EACCES permission denied "styles.less"')
         atom.notifications.onDidAddNotification addErrorHandler = jasmine.createSpy()
 
-      it "creates an error notification", ->
+      it "creates an error notification and does not add the stylesheet", ->
         themeManager.loadUserStylesheet()
         expect(addErrorHandler).toHaveBeenCalled()
         note = addErrorHandler.mostRecentCall.args[0]
         expect(note.getType()).toBe 'error'
         expect(note.getMessage()).toContain 'Error loading'
+        expect(atom.styles.styleElementsBySourcePath[atom.styles.getUserStyleSheetPath()]).toBeUndefined()
 
     describe "when there is an error watching the user stylesheet", ->
       addErrorHandler = null