### Preflight Checklist
* [x] I have read the [Contributing Guidelines](https://github.com/electron/electron/blob/master/CONTRIBUTING.md) for this project.
* [x] I agree to follow the [Code of Conduct](https://github.com/electron/electron/blob/master/CODE_OF_CONDUCT.md) that this project adheres to.
* [x] I have searched the issue tracker for an issue that matches the one I want to file, without success.

### Issue Details
```js
let ret = shell.openItem('file:///noexistent');
console.log(ret);
```
`ret` is always `true` on Linux even if opening fails, e.g.
```sh
xdg-open 'file:///noexistent'
echo $?
```
Shows nonzero error code.

* **Electron Version:**
  * v6.0.11
* **Operating System:**
  *Linux / Ubuntu 16.04 x64

### Expected Behavior
`ret` should be `false`, if `xdg-open` returns non-zero code.

Optional: It would be very nice if the `xdg-open` error (catch stderr) would be displayed in `dialog.showErrorBox` if it returns non-zero code. Just take care that `xdg-open` may write some warnings to stderr even if it returns 0.

shell/browser/ui/inspectable_web_contents_impl.cc
#include "content/public/browser/render_frame_host.h"
#include "content/public/browser/render_view_host.h"
#include "content/public/browser/storage_partition.h"
#include "content/public/common/user_agent.h"
#include "ipc/ipc_channel.h"
#include "net/http/http_response_headers.h"
#include "services/network/public/cpp/simple_url_loader.h"
#include "services/network/public/cpp/simple_url_loader_stream_consumer.h"
#include "shell/browser/ui/inspectable_web_contents_delegate.h"
#include "shell/browser/ui/inspectable_web_contents_view.h"
#include "shell/browser/ui/inspectable_web_contents_view_delegate.h"
#include "shell/common/platform_util.h"
#include "third_party/blink/public/common/logging/logging_utils.h"
#include "third_party/blink/public/common/page/page_zoom.h"
#include "ui/display/display.h"
#include "ui/display/screen.h"
namespace electron {
namespace {
const double kPresetZoomFactors[] = {0.25, 0.333, 0.5,  0.666, 0.75, 0.9,
                                     1.0,  1.1,   1.25, 1.5,   1.75, 2.0,
                                     2.5,  3.0,   4.0,  5.0};
const char kChromeUIDevToolsURL[] =
    "devtools://devtools/bundled/devtools_app.html?"
    "remoteBase=%s&"
    "can_dock=%s&"
    "toolbarColor=rgba(223,223,223,1)&"
    "textColor=rgba(0,0,0,1)&"
    "experiments=true";
const char kChromeUIDevToolsRemoteFrontendBase[] =
    "https://devtools-frontend.appspot.com/";