This sounds similar to Bug 1325876, our error displaying certainly needs to improve. This bug is about improving it in devtools? Improving the shown error page might need a bug in Networking component.
Bug 1842429 Comment 8 Edit History
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This sounds similar to Bug 1325876, our error displaying certainly needs to improve. This bug is about improving it in devtools? Improving the shown error page might need a bug in Networking or DOM: Navigation component.