source/ostree: fix download only case

Source, for compatability reasons, have two modes: download only
and download and export. The difference is the arguments that
are passed to the source: For download only, the `output` param
is empty. In this case also `checksums` *can* be empty and if so
it means everything, i.e. the commits, should be fetched. The
latter was not properly handled so far. Adjust the logic, which
now closely mimics that of the `org.osbuild.curl` source to fix
this case.
Also catch exceptions invoking `ostree` and properly return them
via the json error messaging.

sources/org.osbuild.ostree

@@ -128,14 +128,24 @@ def export(checksums, cache, output):
 
 def main(commits, options, checksums, cache, output):
     cache = os.path.join(cache, "org.osbuild.ostree")
+    download_only = not output
 
     if not commits:
         commits = options.get("commits", {})
 
-    os.makedirs(cache, exist_ok=True)
-    download(commits, checksums, cache)
+    if commits:
+        if not checksums and download_only:
+            checksums = [k for k, _ in commits.items()]
 
-    if not output:
+        os.makedirs(cache, exist_ok=True)
+        try:
+            download(commits, checksums, cache)
+        except subprocess.CalledProcessError as e:
+            output = e.output.strip()
+            json.dump({"error": output}, sys.stdout)
+            return 1
+
+    if download_only:
         json.dump({}, sys.stdout)
         return 0
 
@@ -151,5 +161,5 @@ if __name__ == '__main__':
              source_args["options"],
              source_args["checksums"],
              source_args["cache"],
-             source_args["output"])
+             source_args.get("output"))
     sys.exit(r)