Replace hide/unhide errors with hardcoded strings

The previous code was incorrect because depending on the failure, the
error might not be an `Error`. This needs to be fixed in the API client
but for the moment, sidestep the problem by just displaying a fixed
string.

src/sidebar/components/moderation-banner.js

@@ -32,8 +32,8 @@ function ModerationBannerController(annotationUI, flash, store) {
   this.hideAnnotation = function () {
     store.annotation.hide({id: self.annotation.id}).then(function () {
       annotationUI.hideAnnotation(self.annotation.id);
-    }).catch(function (err) {
-      flash.error(err.message);
+    }).catch(function () {
+      flash.error('Failed to hide annotation');
     });
   };
 
@@ -43,8 +43,8 @@ function ModerationBannerController(annotationUI, flash, store) {
   this.unhideAnnotation = function () {
     store.annotation.unhide({id: self.annotation.id}).then(function () {
       annotationUI.unhideAnnotation(self.annotation.id);
-    }).catch(function (err) {
-      flash.error(err.message);
+    }).catch(function () {
+      flash.error('Failed to unhide annotation');
     });
   };
 }

src/sidebar/components/test/moderation-banner-test.js

@@ -98,7 +98,7 @@ describe('moderationBanner', function () {
     banner.querySelector('button').click();
 
     setTimeout(function () {
-      assert.calledWith(fakeFlash.error, 'Network Error');
+      assert.calledWith(fakeFlash.error, 'Failed to hide annotation');
       done();
     }, 0);
   });
@@ -120,7 +120,7 @@ describe('moderationBanner', function () {
     banner.querySelector('button').click();
 
     setTimeout(function () {
-      assert.calledWith(fakeFlash.error, 'Network Error');
+      assert.calledWith(fakeFlash.error, 'Failed to unhide annotation');
       done();
     }, 0);
   });