Fix error messages about non-existent files

examples/error.feature

@@ -8,3 +8,4 @@ Feature: Error
     Given a file named "foo.md" with ""
     When I run `linkcheck bar.md`
     Then the exit status should be 1
+    And the stderr should contain "no such file"

file_result.go

@@ -1,6 +1,10 @@
 package main
 
-import "strings"
+import (
+	"strings"
+
+	"github.com/fatih/color"
+)
 
 type fileResult struct {
 	filename   string
@@ -11,6 +15,10 @@ type fileResult struct {
 func (r fileResult) String(verbose bool) string {
 	ss := make([]string, 0, len(r.urlResults))
 
+	if r.err != nil {
+		ss = append(ss, "\t"+color.RedString(r.err.Error()))
+	}
+
 	for _, r := range r.urlResults {
 		if r.err != nil || verbose {
 			ss = append(ss, "\t"+r.String())