Report render exceptions separately from data load failures

[caufieldjh]

Fixes #664.

Problem

loadRegistryData() wrapped both the fetch and the render in a single .catch(showLoadError), so any exception thrown while rendering successfully fetched data — including the deliberate "Registry data is missing its list of resources" guard — was presented to the user as a network load failure, with a Retry button that fails identically every time. This undercut the distinct error state introduced for #660/#661.

Fix

• loadRegistryData() now uses the two-argument then(onFulfilled, onRejected) form: a rejected fetch goes to showLoadError as before, while onRegistryDataLoaded runs inside its own try/catch so render exceptions can no longer reach the load-error handler.
• New showRenderError() shows a distinct alert: the data loaded, but an error occurred while displaying it, so retrying will not help. It includes the underlying error message and a "Report this problem" link to the issue tracker instead of a Retry button. Badges get the same — error treatment as a load failure.

Verification

Exercised end-to-end in headless Chromium against the built site, using the reproduction from the issue:

• Malformed data (HTTP 200, category is a number): shows "Could not display the registry data" with Details: r.category.toLowerCase is not a function and the report link; badges read — resources / — KGs. Previously this showed the load-failure banner.
• Missing resources key (valid JSON): shows the display error with "Registry data is missing its list of resources".
• 404 on both data URLs: still shows "Could not load the registry data" with the Retry button.
• Real summary data: table renders normally (1012 resources / 153 KGs), no error state.

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