fix(webview): hide progress bar on main-frame load errors

[jim-daf]

Closes #35.

ProgressBarWebViewClient shows the progress bar on onPageStarted and hides it on onPageFinished. On some failure modes the WebView reports the failure through onReceivedError and skips onPageFinished, so the spinner stays visible over a blank WebRegisterActivity / PrivacyPolicyActivity. The user has no way to know the page never loaded.

Add a main-frame-only onReceivedError override that hides the progress bar too:

@Override
public void onReceivedError(WebView view, WebResourceRequest request,
                            WebResourceError error) {
    super.onReceivedError(view, request, error);
    if (request != null && request.isForMainFrame()) {
        mProgressBar.setVisibility(View.GONE);
    }
}

isForMainFrame keeps a single blocked sub-resource from hiding the spinner while the main load is still running. No callers of the client are affected.