Propagate backend errors up to popup

[urfeex]

This adds bringing information about why fetching the script code didn't work back to the user. This will let the user know whether the port could not be reached, querying data timed out or if an empty response was received.

This builds ontop of #30, hence the draft status

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Note

Medium Risk
Changes error-handling paths for robot connectivity and user-facing validation without altering successful fetch logic; misclassified errno or HTTP status could confuse troubleshooting but impact is limited to error messaging.

Overview
Improves visibility when remote script fetch fails by mapping robot connectivity problems to meaningful HTTP responses and surfacing those messages in the URCap UI and generated program popups.

The Flask REST API now returns non-200 statuses (e.g. 504 for timeouts, 503 for unreachable/refused connections with extra guidance on connection refused) while still embedding status in the JSON body. RequestProgram.send_command no longer wraps all failures in a generic connectivity message; receive timeouts raise TimeoutError and empty responses use clearer error text so the API layer can classify them.

On the frontend, failed backend fetches include the JSON status in errors shown via validation and script-generation popup. saveNode runs validation after update and opens a confirm dialog when invalid, with new i18n strings for the failure title and message.

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