Print error on formatter failure (#235)

It'd necessary to know why code formatters fail, so we now print the
full output from configured tooling in case of error. Some formatters
use stderr and some use stdout, so we join both of them in that message.

openapi/gen/main.go

@@ -2,6 +2,7 @@
 package main
 
 import (
+	"bytes"
 	"encoding/json"
 	"errors"
 	"flag"
@@ -152,11 +153,22 @@ func (r *Render) Run() error {
 		formatter = strings.ReplaceAll(formatter, "$FILENAMES",
 			strings.Join(filenames, " "))
 		split := strings.Split(formatter, " ")
+
+		// create pipe to forward stdout and stderr to same fd,
+		// so that it's clear why formatter failed.
+		reader, writer := io.Pipe()
+		out := bytes.NewBuffer([]byte{})
+		go io.Copy(out, reader)
+		defer reader.Close()
+		defer writer.Close()
+
 		cmd := exec.Command(split[0], split[1:]...)
 		cmd.Dir = r.ctx.Target
+		cmd.Stdout = writer
+		cmd.Stderr = writer
 		err := cmd.Run()
 		if err != nil {
-			return fmt.Errorf("%s: %w", formatter, err)
+			return fmt.Errorf("%s:\n%s", formatter, out.Bytes())
 		}
 	}
 	return nil