change 'CoercedVariableValues' to disjoint union

Replicates graphql/graphql-js@168570f

Author: Cito

src/graphql/execution/execute.py

@@ -278,20 +278,16 @@ def build(
             raw_variable_values or {},
         )
 
-        if coerced_variable_values.errors:
-            return coerced_variable_values.errors
-
-        variable_values = coerced_variable_values.coerced
-        if variable_values is None:
-            raise TypeError("Has variables if no errors.")
+        if isinstance(coerced_variable_values, list):
+            return coerced_variable_values  # errors
 
         return cls(
             schema,
             fragments,
             root_value,
             context_value,
             operation,
-            variable_values,
+            coerced_variable_values,  # coerced values
             field_resolver or default_field_resolver,
             type_resolver or default_type_resolver,
             [],

src/graphql/execution/values.py

@@ -1,4 +1,4 @@
-from typing import Any, Dict, List, NamedTuple, Optional, Union, cast
+from typing import Any, Dict, List, Optional, Union, cast
 
 from ..error import GraphQLError, INVALID
 from ..language import (
@@ -28,9 +28,7 @@
 __all__ = ["get_variable_values", "get_argument_values", "get_directive_values"]
 
 
-class CoercedVariableValues(NamedTuple):
-    errors: Optional[List[GraphQLError]]
-    coerced: Optional[Dict[str, Any]]
+CoercedVariableValues = Union[List[GraphQLError], Dict[str, Any]]
 
 
 def get_variable_values(
@@ -105,11 +103,7 @@ def get_variable_values(
 
         coerced_values[var_name] = coerced.value
 
-    return (
-        CoercedVariableValues(errors, None)
-        if errors
-        else CoercedVariableValues(None, coerced_values)
-    )
+    return errors or coerced_values
 
 
 def get_argument_values(