composer-cli: Handle download errors

The API will return an error 400 with a json error object if the
download fails, catch this and report the error to the user without a
traceback.

src/composer/cli/__init__.py

@@ -49,4 +49,8 @@ def main(opts):
         log.error("Missing %s sub-command", opts.args[0])
         return 1
     else:
-        return command_map[opts.args[0]](opts)
+        try:
+            return command_map[opts.args[0]](opts)
+        except Exception as e:
+            log.error(str(e))
+            return 1

src/composer/http_client.py

@@ -158,6 +158,10 @@ def download_file(socket_path, url, progress=True):
     """
     http = UnixHTTPConnectionPool(socket_path)
     r = http.request("GET", url, preload_content=False)
+    if r.status == 400:
+        err = json.loads(r.data.decode("utf-8"))
+        if not err["status"]:
+            raise RuntimeError(err["error"]["msg"])
 
     filename = get_filename(r)
     if os.path.exists(filename):