add function to test retrieval of WPAD file in proxy settings

[Karl-Philipp Richter]

I'd suggest to add a "test retrieval" and "test retrieval and display" button. It's crucial to provide useful feedback in case of failure including the HTTP error code or others error messages from Linux ("host not reachable"), invalid URL, etc.

ProblemType: Bug
DistroRelease: Ubuntu 15.04
Package: unity 7.3.2+15.04.20150420-0ubuntu1
Uname: Linux 4.0.1-040001-generic x86_64
ApportVersion: 2.17.2-0ubuntu1
Architecture: amd64
CompizPlugins: »/apps/compiz-1/general/screen0/options/active_plugins« wurde kein Wert zugewiesen
CurrentDesktop: Unity
Date: Thu May 14 18:44:59 2015
EcryptfsInUse: Yes
InstallationDate: Installed on 2015-04-02 (41 days ago)
InstallationMedia: Ubuntu 15.04 "Vivid Vervet" - Beta amd64 (20150326)
ProcEnviron:
 TERM=xterm
 PATH=(custom, no user)
 XDG_RUNTIME_DIR=<set>
 LANG=de_DE.UTF-8
 SHELL=/bin/bash
SourcePackage: unity
SystemImageInfo:
 current build number: 0
 device name:
 channel: daily
 last update: Unknown
UpgradeStatus: No upgrade log present (probably fresh install)