Replace errors pkg with fmt.Errorf (#27)

templates/client.mustache

@@ -6,7 +6,6 @@ import (
 	"context"
 	"encoding/json"
 	"encoding/xml"
-	"errors"
 	"fmt"
 	"io"
 	"log"
@@ -314,7 +313,7 @@ func (c *APIClient) prepareRequest(
 	// add form parameters and file if available.
 	if strings.HasPrefix(headerParams["Content-Type"], "multipart/form-data") && len(formParams) > 0 || (len(formFiles) > 0) {
 		if body != nil {
-			return nil, errors.New("Cannot specify postBody and multipart form at the same time.")
+			return nil, fmt.Errorf("cannot specify postBody and multipart form at the same time.")
 		}
 		body = &bytes.Buffer{}
 		w := multipart.NewWriter(body)
@@ -355,7 +354,7 @@ func (c *APIClient) prepareRequest(
 
 	if strings.HasPrefix(headerParams["Content-Type"], "application/x-www-form-urlencoded") && len(formParams) > 0 {
 		if body != nil {
-			return nil, errors.New("Cannot specify postBody and x-www-form-urlencoded form at the same time.")
+			return nil, fmt.Errorf("cannot specify postBody and x-www-form-urlencoded form at the same time.")
 		}
 		body = &bytes.Buffer{}
 		body.WriteString(formParams.Encode())
@@ -521,14 +520,14 @@ func (c *APIClient) decode(v interface{}, b []byte, contentType string) (err err
 					return err
 				}
 			} else {
-				return errors.New("Unknown type with GetActualInstance but no unmarshalObj.UnmarshalJSON defined")
+				return fmt.Errorf("unknown type with GetActualInstance but no unmarshalObj.UnmarshalJSON defined")
 			}
 		} else if err = json.Unmarshal(b, v); err != nil { // simple model
 			return err
 		}
 		return nil
 	}
-	return errors.New("undefined response type")
+	return fmt.Errorf("undefined response type")
 }
 
 // Add a file to the multipart request