showPreview option is not covered in all cases

[BakasuraRCE]

Prerequisites

• I have searched for similar issues in both open and closed tickets and cannot find a duplicate.
• The issue still exists against the latest master branch of bootstrap-fileinput.
• This is not an usage question. I confirm having read the plugin documentation and demos.
• This is not a general programming / coding question. (Those should be directed to the webtips Q & A forum).
• I have attempted to find the simplest possible steps to reproduce the issue.
• I have included a failing test as a pull request (Optional).

Steps to reproduce the issue

1. Set showPreview to false
2. Upload any file
3. Return an error response in the server
4. The error happend

Expected behavior and actual behavior

In the OnSuccess event the variable $thumb is not defined when showPreview are disabled:

bootstrap-fileinput/js/fileinput.js

Line 2110 in 5911e2c

self._setPreviewError($thumb, i, self.filestack[i], self.retryErrorUploads);

This happens too in:
#1082

Maybe this is happening in other parts of the code.

Temporal Fix:

1. Set showPreview to true

2. Null the preview template

   'layoutTemplates': {
       'preview': ''
   },
   

3. Define one custom class for elErrorContainer