sources(curl): error if curl exists 0 but there are downloads left

As part of the investigation of the CI failure in
https://github.com/osbuild/osbuild-composer/pull/4247
we noticed that curl can return a return_code of `0` even
when it did not downloaded all the urls in a `--config` provided
file. This seems to be curl version dependent, I had a hard
time writing a test-case with the real curl (8.6.0) that
reproduces this so I went with mocking it. We definietly saw
this failure with the centos 9 version (7.76).

Our current code is buggy and assumes that the exit status
of curl is always non-zero if any download fails but that is
only the case when `--fail-early` is used.

The extra paranoia will not hurt even when relying on the
exit code of curl is fixed.

sources/org.osbuild.curl

@@ -317,6 +317,9 @@ class CurlSource(sources.SourceService):
                 failed_urls = ",".join([itm[1]["url"] for itm in dl_pairs])
                 raise RuntimeError(f"curl: error downloading {failed_urls}: error code {return_code}")
 
+        if len(dl_pairs) > 0:
+            raise RuntimeError(f"curl: finished with return_code {return_code} but {dl_pairs} left to download")
+
     def _fetch_all_old_curl(self, amended):
         with concurrent.futures.ThreadPoolExecutor(max_workers=self.max_workers) as executor:
             for _ in executor.map(self.fetch_one, *zip(*amended)):