refine executor bug info (#19887)

test=develop

python/paddle/fluid/executor.py

@@ -621,11 +621,15 @@ class Executor(object):
         if self._closed:
             raise RuntimeError("Attempted to use a closed Executor")
 
+        use_default_main_program = program is None
         if program is None:
             program = default_main_program()
-        if isinstance(program,Program) and \
+        if isinstance(program, Program) and \
                         len(program.global_block().ops) == 0:
-            warnings.warn("The current program is empty.")
+            error_info = "The current program is empty."
+            if use_default_main_program:
+                error_info += " Maybe you should pass the Program or the CompiledProgram manually."
+            warnings.warn(error_info)
 
         if scope is None:
             scope = global_scope()