added BufferedJSON.exists() method, use it in get_time/get_data

dap/utils/bufjson.py

@@ -25,11 +25,20 @@ class BufferedJSON:
 
 
     def get_time(self):
+        if not self.exists():
+            return -1
         return os.path.getmtime(self.fname)
 
     def get_data(self, *args, **kwargs):
+        if not self.exists():
+            return {}
         return json_load(self.fname, *args, **kwargs)
 
+    def exists(self):
+        if not self.fname:
+            return False
+        return os.path.exists(self.fname)
+
 
 
 def json_load(filename, *args, **kwargs):