Minor touch ups after #342

Author: leebyron

src/execution/execute.js

@@ -818,18 +818,17 @@ function completeAbstractValue(
     defaultResolveTypeFn(result, exeContext.contextValue, info, returnType);
 
   invariant(
-    !isNullish(runtimeType),
+    !runtimeType,
     `Could not determine runtime type of value "${result}" for field ${
       info.parentType}.${info.fieldName}.`
   );
   invariant(
     runtimeType instanceof GraphQLObjectType,
-    `resolveType must return an instance of GraphQLObjectType for field ${
-      info.parentType}.${info.fieldName}, received "${runtimeType}".`
+    `${returnType}.resolveType must return an instance of GraphQLObjectType `+
+    `for field ${info.parentType}.${info.fieldName}, received "${runtimeType}".`
   );
 
-  const schema = exeContext.schema;
-  if (!schema.isPossibleType(returnType, runtimeType)) {
+  if (!exeContext.schema.isPossibleType(returnType, runtimeType)) {
     throw new GraphQLError(
       `Runtime Object type "${runtimeType}" is not a possible type ` +
       `for "${returnType}".`,