command: move utility functions to print errors to streams package

Move the implementation to print errors from the command/helpers.go file to the
streams package.
It fits perfectly there.

internal/command/helpers.go

@@ -6,8 +6,6 @@ import (
 	"os"
 	"path/filepath"
 
-	"github.com/fatih/color"
-
 	"github.com/simplesurance/baur/v2/internal/command/term"
 	"github.com/simplesurance/baur/v2/internal/format"
 	"github.com/simplesurance/baur/v2/internal/log"
@@ -217,25 +215,21 @@ func mustWriteRow(fmt format.Formatter, row ...interface{}) {
 	exitOnErr(err)
 }
 
-var errorPrefix = color.New(color.FgRed).Sprint("ERROR:")
-
 func exitOnErrf(err error, format string, v ...interface{}) {
-	exitOnErr(err, fmt.Sprintf(format, v...))
+	if err == nil {
+		return
+	}
+
+	stderr.ErrPrintf(err, format, v...)
+	exitFunc(1)
 }
 
 func exitOnErr(err error, msg ...interface{}) {
 	if err == nil {
 		return
 	}
 
-	if len(msg) == 0 {
-		stderr.Println(errorPrefix, err)
-		exitFunc(1)
-	}
-
-	wholeMsg := fmt.Sprint(msg...)
-	stderr.Printf("%s %s: %s\n", errorPrefix, wholeMsg, err)
-
+	stderr.ErrPrintln(err, msg...)
 	exitFunc(1)
 }
 

internal/command/term/streams.go

@@ -5,11 +5,15 @@ import (
 	"io"
 	"sync"
 
+	"github.com/fatih/color"
+
 	"github.com/simplesurance/baur/v2/pkg/baur"
 )
 
 const separator = "------------------------------------------------------------------------------"
 
+var errorPrefix = color.New(color.FgRed).Sprint("ERROR:")
+
 // Stream is a concurrency-safe output for term.messages.
 type Stream struct {
 	stream io.Writer
@@ -41,6 +45,24 @@ func (s *Stream) TaskPrintf(task *baur.Task, format string, a ...interface{}) {
 	s.Printf(prefix+format, a...)
 }
 
+// ErrPrintln prints an error with an optional message.
+// The method prints the error in the format: errorPrefix msg: err
+func (s *Stream) ErrPrintln(err error, msg ...interface{}) {
+	if len(msg) == 0 {
+		s.Println(errorPrefix, err)
+		return
+	}
+
+	wholeMsg := fmt.Sprint(msg...)
+	s.Printf("%s %s: %s\n", errorPrefix, wholeMsg, err)
+}
+
+// ErrPrintf prints an error with an optional printf-style message.
+// The method prints the error in the format: errorPrefix msg: err
+func (s *Stream) ErrPrintf(err error, format string, a ...interface{}) {
+	s.ErrPrintln(err, fmt.Sprintf(format, a...))
+}
+
 // PrintSep prints a separator line
 func (s *Stream) PrintSep() {
 	fmt.Fprintln(s.stream, separator)