Indeed, <noscript> doesn't show just because the page didn't properly load the scripts in the page. It's not a fallback for errors, it's a fallback to serve users who deliberately disabled Javascript. This is a rare scenario these days, but it does get displayed when you disable JS in Tor Browser, use the disable Javascript button in uBlock Origin (I personally use this to whitelist javascript per-domain), or use various other extensions like NoScript. This is dependent on the implementation, though. In theory some crappy browser extension could provide JS disabling functionality otherwise identical to tor/ublock/noscript but forget to display <noscript>s, but I haven't heard of implementations that are like this.

Either way, make sure you have something sensible to display for all scenarios, even if it's just an error page. Mysterious blank pages are not fun.