GUI: allow to use custom re-captcha public key

- Read public key for Re-Captcha from perun-web-gui.properties
  stored on server side. This will allow us to use it in
  different Perun instances without need to share are private
  key part.

- If key not present, stop loading appForm gui and display error message.

- Updated PerunError object, so more properties can be set manually.

- Small updates to error reporting widget.

- When response on POST is null (error case) and callback is hidden,
  do not show error report widget.
Merge pull request #244 from zlamalp/captcha

GUI: allow to use custom re-captcha public key