GUI: Handle also 403 error code

- When contacting Perun, handle also 403 Forbidden, which happens
  when you have valid session, but with different server or your
  session is expired but browser uses cache.
Merge pull request #623 from zlamalp/forbidden

GUI: Handle also 403 error code
GUI: Proper handling of all HTTP return codes

- Display proper error message on any http return code != 200.
  Our special cases are handled as of until now.
- Code simplification.