Write usage message to stderr on error

When the parsing of parameters/flags fails eg. when a required flag is
missing, print the usage statement and error to stderr instead of
stdout.

usage.go

@@ -9,9 +9,9 @@ import (
 	"strings"
 )
 
-// Fail prints usage information to stdout and exits with non-zero status
+// Fail prints usage information to stderr and exits with non-zero status
 func (p *Parser) Fail(msg string) {
-	p.WriteUsage(os.Stdout)
+	p.WriteUsage(os.Stderr)
 	fmt.Println("error:", msg)
 	os.Exit(-1)
 }