message: Fileviewer: center error etc. messages vertically
diff: @@ -343,6 +343,7 @@ void FileViewer::ShowError(const char *icon, const wxString& msg, const wxString
  <style>%s</style>
  </head>
  <body>
  + <div class="message-wrapper">
  <div class="message">
  <h1>%s</h1>
  <h2>%s</h2>
  @@ -360,6 +361,7 @@ void FileViewer::ShowError(const char *icon, const wxString& msg, const wxString
  html += R"(
  </div>
  + </div>
  </body>
  </html>
  )";
  @@ -746,6 +748,13 @@ mark {
  /* Error messages: */
  +.message-wrapper {
  + height: 90vh;
  + display: flex;
  + align-items: center;
  + justify-content: center;
  +}
  +
  .message {
  text-align: center;
  opacity: 0.8;
  