WebxdcStoreActivity: log main-frame WebView load errors

[jim-daf]

Closes #153.

WebxdcStoreActivity loads a remote store URL with webView.loadUrl(Prefs.getWebxdcStoreUrl(this)). The existing WebViewClient overrides shouldOverrideUrlLoading and shouldInterceptRequest, but not onReceivedError, so WebView-layer load failures end up silent in adb logcat.

interceptRequest already catches RpcException and friends and synthesises a "Could not load apps. Are you online?" plain-text body, which is good UX for the offline case. The change here is strictly additive: a Log.w line that fires for request.isForMainFrame() errors only, so the next time someone reports "the store screen is blank" there is something in the log to go on.

@Override
@RequiresApi(Build.VERSION_CODES.M)
public void onReceivedError(WebView view, WebResourceRequest request, WebResourceError error) {
    super.onReceivedError(view, request, error);
    if (request.isForMainFrame()) {
        Log.w(TAG, "Store load failed: " + error.getErrorCode() + " " + error.getDescription());
    }
}

Notes:

• API 23+ overload only. isForMainFrame() and WebResourceError are both API 23, and the project already uses @RequiresApi(21) on sibling overrides in the same class.
• Sub-resource errors are filtered out by the isForMainFrame() guard so the log stays useful.
• Uses the Log import that is already present in this file, and the TAG constant that is already defined on the class. No new permission, no new dependency, no behaviour change for successful loads.