Closes #11029.

K9WebViewClient already overrides shouldOverrideUrlLoading, shouldInterceptRequest and onPageFinished. It does not override onReceivedError, so any failure while loading a message body in the legacy message view is silent: the user sees a blank message body, and nothing useful gets recorded in adb logcat.

This PR adds the API 23+ overload only, since minSdk for the legacy module is 23 (build-plugin/src/main/kotlin/ThunderbirdProjectConfig.kt: const val sdkMin = 23), so the deprecated pre-M overload is not reachable on any device the app ships to.

@RequiresApi(Build.VERSION_CODES.M)
override fun onReceivedError(view: WebView, request: WebResourceRequest, error: WebResourceError) {
    super.onReceivedError(view, request, error)
    if (request.isForMainFrame) {
        Log.d("Message WebView load error: %d %s for %s", error.errorCode, error.description, request.url)
    }
}

Notes on scope:

• Only main-frame errors are logged. Sub-resource errors are very common in HTML mails (broken remote images, blocked http:// resources inside an https:// mail) and would just spam the log without giving useful information about the root render failure.
• The log call uses Log.d via net.thunderbird.core.logging.legacy.Log, matching the existing imports and call style in this file (see the existing Log.d(e, "Couldn't open URL: %s", uri) on the same class).
• No new permission, no new dependency, no behaviour change for successful loads. The change is additive: an existing silent failure now produces a debug log line.

Out of scope but mentioned in the issue: QuotedMessageMvpView has the same gap. Happy to send a follow-up PR if maintainers prefer that one bundled, but K9WebViewClient is the dominant codepath and felt like the right place to start.