Fix error handling for safari

Author: timsuchanek

src/types.ts

@@ -43,7 +43,10 @@ export class ClientError extends Error {
     this.response = response
     this.request = request
 
-    Error.captureStackTrace(this, ClientError)
+    // this is needed as Safari doesn't support .captureStackTrace
+    if (typeof Error.captureStackTrace === 'function') {
+      Error.captureStackTrace(this, ClientError)
+    }
   }
 
   private static extractMessage (response: GraphQLResponse): string {