luci-base: runtime.uc: fix error500() invocation on theme failure

In case no single theme could be loaded successfully, the runtime class
failed to properly invoke the `error500()` method, which is only available
through the passed environment at this point.

Signed-off-by: Jo-Philipp Wich <jo@mein.io>

modules/luci-base/ucode/runtime.uc

@@ -173,7 +173,7 @@ export default function(env) {
 		}
 
 		if (!media)
-			error500(`Unable to render any theme header template, last error was:\n${status}`);
+			env.dispatcher.error500(`Unable to render any theme header template, last error was:\n${status}`);
 	}
 
 	self.env.media = media;