Merge branch '19161-non-helpful-flash-message-when-attaching-file-fails' into 'master'

Hacked in a better message for attachment network error

## What does this MR do?

Adds a better error message for when a user is uploading an attachment and they cannot reach the GitLab instance.

Now displays `Network error. Please check your connection.`.

## Are there points in the code the reviewer needs to double check?

As noted above, I think this is hacky, ideally this would be sent straight from the backend but if not then this should be handled globally?

/cc @fatihacet 

## Why was this MR needed?

Attachment upload UX

## What are the relevant issue numbers?

Closes #19161.

## Screenshots (if relevant)

*UPDATED:*

![Screen_Shot_2016-06-30_at_00.42.37](/uploads/b3b4044782dd4c4c17735e99acc45502/Screen_Shot_2016-06-30_at_00.42.37.png)

## Does this MR meet the acceptance criteria?

- [ ] [CHANGELOG](https://gitlab.com/gitlab-org/gitlab-ce/blob/master/CHANGELOG) entry added
- [ ] [Documentation created/updated](https://gitlab.com/gitlab-org/gitlab-ce/blob/master/doc/development/doc_styleguide.md)
- [ ] API support added
- Tests
  - [ ] Added for this feature/bug
  - [ ] All builds are passing
- [ ] Conform by the [style guides](https://gitlab.com/gitlab-org/gitlab-ce/blob/master/CONTRIBUTING.md#style-guides)
- [ ] Branch has no merge conflicts with `master` (if you do - rebase it please)
- [ ] [Squashed related commits together](https://git-scm.com/book/en/Git-Tools-Rewriting-History#Squashing-Commits)

See merge request !4977

app/assets/javascripts/dropzone_input.js.coffee

@@ -70,12 +70,12 @@ class @DropzoneInput
         pasteText response.link.markdown
         return
 
-      error: (temp, errorMessage) ->
+      error: (temp) ->
         errorAlert = $(form).find('.error-alert')
         checkIfMsgExists = errorAlert.children().length
         if checkIfMsgExists is 0
           errorAlert.append divAlert
-          $(".div-dropzone-alert").append btnAlert + errorMessage
+          $(".div-dropzone-alert").append "#{btnAlert}Attaching the file failed."
         return
 
       totaluploadprogress: (totalUploadProgress) ->