Fetches the error message when manifest upload fails.

Uploading manifest seems to fail frequently when uploading at the
jenkins end. This will help us know the exact error.
Here it is an example:

```
======================================================================
FAIL: @test: attempt to publish a content view containing RH content
----------------------------------------------------------------------
Traceback (most recent call last):
  File "/home/kedar/git_world/robottelo/tests/foreman/ui/test_contentviews.py", line 805, in test_cv_publish_rh_1
    self.setup_to_create_cv(session, cv_name, rh_repo=rh_repo)
  File "/home/kedar/git_world/robottelo/tests/foreman/ui/test_contentviews.py", line 76, in setup_to_create_cv
    u'Following is the error: {0}'.format(task['humanized']['errors']))
AssertionError: u'success' != u'warning'
- success
+ warning
 : Following is the error: [u'Archive failed signature check']

----------------------------------------------------------------------
XML: foreman-results.xml
----------------------------------------------------------------------
Ran 1 test in 137.596s
```