no error reported if skill manifest can not be accessed

[hibrenda]

• connect to a new skill
• input the url which can not be accessed:
  
• no error reported but get following empty line
  

Sometimes, I add the new skill through the drop down list.

it showes empty but actually it is not empty as the url has already been added.

[cwhitten]

to fixer: when someone inputs a manifest url, fetch the url and make sure we get a successful HTTP 200 back before accepting it as a manifest