Move errors to gemini.go

fs.go

@@ -1,7 +1,6 @@
 package gmi
 
 import (
-	"errors"
 	"io"
 	"mime"
 	"os"
@@ -15,11 +14,6 @@ func init() {
 	mime.AddExtensionType(".gemini", "text/gemini")
 }
 
-// FileServer errors.
-var (
-	ErrNotAFile = errors.New("gemini: not a file")
-)
-
 // FileServer takes a filesystem and returns a Handler which uses that filesystem.
 // The returned Handler sanitizes paths before handling them.
 func FileServer(fsys FS) Handler {