Extension upgrade: network outage silently makes extensions appear delisted

## Description

When the network is down during `azd extension upgrade --all`, SourceManager silently drops unreachable sources and `FindExtensions` returns 0 matches with no error. The `isNetworkError` check never fires because `err` is nil. This causes every extension to appear "delisted" rather than surfacing the actual network failure.

## Expected Behavior

When a source is unreachable due to network issues, the error should be surfaced so `isNetworkError` can detect it and report the actual problem to the user, rather than silently treating all extensions as delisted.

## Context

Identified during review of PR #7853 by @jongio.

## Suggested Approach

Surface the underlying error from SourceManager when a source fails to load, rather than silently dropping it. This would allow the upgrade flow to distinguish between "extension genuinely removed from registry" and "registry unreachable."