cpe_remote_file: fail with clear message on invalid source url

Summary: cpe_remote_file builds its source URL from gen_url (which can delegate to the cpe distro API). That lookup can return nil when the asset/handle cannot be resolved. A nil/invalid source makes the inner remote_file raise an opaque Chef::Exceptions::InvalidRemoteFileURI when the resource is BUILT (the source setter validates), which is before the resource’s own only_if guard can run – so the run crashes with a message that does not say why.

This is the top remote_file exception family (InvalidRemoteFileURI, ~67 devices/day across cpe_waiter, cpe_fonts, cpe_wallpaper, cpe_nudge, cpe_uber_agent, cpe_gk, …).

Validate the source URL with valid_url? BEFORE declaring the remote_file (so we pre-empt the opaque build-time exception) and fail with an explicit, actionable message instead. The message names the resource, file, target path, and root cause – distinguishing an unresolved url (gen_url/cpe distro returned nil) from an unreachable one (a HEAD request failed). Drops the now-redundant inner only_if.

Per reviewer feedback (dtheyer): a resource that cannot converge system state to the specified intent should hard-fail rather than silently skip. This replaces the earlier skip-with-warn approach with a clear hard failure.

Reviewed By: dtheyer

Differential Revision: D109486793

fbshipit-source-id: 73f3c88d37f1f4d9e5f617d6be9582c0d671e7bf