js/main: Throw error if no valid default stylesheet is found

Throw an error using an informative message in case a mode uses a stylesheet
that can't be loaded, instead of crashing later because the theming can't be
properly computed, and thus the minimum size of the actors.

js/ui/main.js

@@ -343,6 +343,9 @@ function loadTheme() {
     let theme = new St.Theme ({ application_stylesheet: _cssStylesheet,
                                 default_stylesheet: _defaultCssStylesheet });
 
+    if (theme.default_stylesheet == null)
+        throw new Error("No valid stylesheet found for '%s'".format(sessionMode.stylesheetName));
+
     if (previousTheme) {
         let customStylesheets = previousTheme.get_custom_stylesheets();