added dm.test_dfs() for DFS availability

csst_common/data_manager.py

@@ -460,11 +460,20 @@ class CsstMsDataManager:
         try:
             tbl = gaia_query_from_dfs(
                 ra=ra, dec=dec, radius=radius, min_mag=min_mag, max_mag=max_mag, obstime=obstime, limit=limit)
-            assert len(tbl) > 0
             return tbl
-        except Exception as e:
+        except:
+            print("Error occurred during the query!")
             return None
 
+    def test_dfs(self):
+        """ Test if DFS works. """
+        try:
+            tbl = gaia_query_from_dfs(radius=0.001)
+            assert len(tbl) == 0
+            return True
+        except:
+            return False
+
     def __repr__(self):
         lines = ""
         lines += "<CsstMsDataManager>\n"