Improve error messages, esp. when the server is running

* Add file context to minifier errors when publishing
* Misc fixes (see issues)
* Allow custom server error template in layouts/server/error.html

To get to this, this commit also cleans up and simplifies the code surrounding errors and files. This also removes the usage of `github.com/pkg/errors`, mostly because of https://github.com/pkg/errors/issues/223 -- but also because most of this is now built-in to Go.

Fixes #9852
Fixes #9857
Fixes #9863

commands/new_site.go

@@ -16,14 +16,13 @@ package commands
 import (
 	"bytes"
 	"errors"
+	"fmt"
 	"path/filepath"
 	"strings"
 
 	"github.com/gohugoio/hugo/config"
 	"github.com/gohugoio/hugo/parser/metadecoders"
 
-	_errors "github.com/pkg/errors"
-
 	"github.com/gohugoio/hugo/create"
 	"github.com/gohugoio/hugo/helpers"
 	"github.com/gohugoio/hugo/hugofs"
@@ -94,7 +93,7 @@ func (n *newSiteCmd) doNewSite(fs *hugofs.Fs, basepath string, force bool) error
 
 	for _, dir := range dirs {
 		if err := fs.Source.MkdirAll(dir, 0777); err != nil {
-			return _errors.Wrap(err, "Failed to create dir")
+			return fmt.Errorf("Failed to create dir: %w", err)
 		}
 	}