added some error handling help

Author: matryer

README.md

@@ -5,11 +5,12 @@ Low-level GraphQL client for Go.
 * Simple, familiar API
 * Respects context.Context cancallation
 * Build and execute any kind of GraphQL request
+* Use variables and upload files
 
 ```go
 ctx := context.Background()
 ctx := graphql.NewContext(ctx, "https://machinebox.io/graphql")
-r := graphql.NewRequest(`
+req := graphql.NewRequest(`
     query ($key: String!) {
         items (id:$key) {
             field1
@@ -18,9 +19,9 @@ r := graphql.NewRequest(`
         }
     }
 `)
-r.Var("key", "value")
+req.Var("key", "value")
 var res ResponseStruct
-if err := r.Run(ctx, &resp); err != nil {
+if err := req.Run(ctx, &resp); err != nil {
     log.Fatalln(err)
 }
 ```

client.go

@@ -127,7 +127,11 @@ func NewRequest(q string) *Request {
 	return req
 }
 
-// Run executes the query and unmarshals the response into response.
+// Run executes the query and unmarshals the response from the data field
+// into the response object.
+// Pass in a nil response object to skip response parsing.
+// If the request fails or the server returns an error, the first error
+// will be returned. Use IsGraphQLErr to determine which it was.
 func (req *Request) Run(ctx context.Context, response interface{}) error {
 	client, err := fromContext(ctx)
 	if err != nil {
@@ -152,6 +156,13 @@ func (req *Request) File(filename string, r io.Reader) {
 	})
 }
 
+// IsGraphQLErr gets whether the error is a remote GraphQL
+// server error or not.
+func IsGraphQLErr(err error) bool {
+	_, ok := err.(graphErr)
+	return ok
+}
+
 // file represents a file to upload.
 type file struct {
 	Name string