lib/fetcher: Factor out HTTP status code handling from soup and curl

Use the same G_IO_ERROR_* values for HTTP status codes in both fetchers.
The libsoup fetcher still handles a few more internal error codes than
the libcurl one; this could be built on in future.

Signed-off-by: Philip Withnall <withnall@endlessm.com>

Closes: #1594
Approved by: jlebon

src/libostree/ostree-fetcher-util.h

@@ -78,6 +78,8 @@ void _ostree_fetcher_journal_failure (const char *remote_name,
 gboolean _ostree_fetcher_should_retry_request (const GError *error,
                                                guint         n_retries_remaining);
 
+GIOError _ostree_fetcher_http_status_code_to_io_error (guint status_code);
+
 G_END_DECLS
 
 #endif