Update error message when invalid sympy arg is serialized (#5487)

Fixes #2384

Clarifies what sympy expressions are allowed when a user tries to serialize a circuit which contains unrecognized Sympy expressions.

@dstrain115

cirq-google/cirq_google/serialization/arg_func_langs.py

@@ -190,7 +190,11 @@ def check_support(func_type: str) -> str:
         for arg in value.args:
             arg_to_proto(arg, arg_function_language=arg_function_language, out=msg.func.args.add())
     else:
-        raise ValueError(f'Unrecognized arg type: {type(value)}')
+        raise ValueError(
+            f"Unrecognized Sympy expression type: {type(value)}."
+            " Only the following types are recognized: 'sympy.Symbol', 'sympy.Add', 'sympy.Mul',"
+            " 'sympy.Pow'."
+        )
 
 
 def float_arg_from_proto(