[tune] Fully deprecate raw function literals in Tune (#3788)

Related: https://github.com/ray-project/ray/issues/3785

python/ray/tune/suggest/variant_generator.py

@@ -238,8 +238,8 @@ def _is_resolved(v):
 
 def _try_resolve(v):
     if isinstance(v, types.FunctionType):
-        logger.warning(
-            "Deprecation warning: Function values are ambiguous in Tune "
+        raise DeprecationWarning(
+            "Function values are ambiguous in Tune "
             "configuations. Either wrap the function with "
             "`tune.function(func)` to specify a function literal, or "
             "`tune.sample_from(func)` to tell Tune to "