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

hugolib/fileInfo.go

@@ -14,12 +14,11 @@
 package hugolib
 
 import (
+	"fmt"
 	"strings"
 
 	"github.com/gohugoio/hugo/hugofs/files"
 
-	"github.com/pkg/errors"
-
 	"github.com/gohugoio/hugo/hugofs"
 
 	"github.com/spf13/afero"
@@ -41,7 +40,7 @@ type fileInfo struct {
 func (fi *fileInfo) Open() (afero.File, error) {
 	f, err := fi.FileInfo().Meta().Open()
 	if err != nil {
-		err = errors.Wrap(err, "fileInfo")
+		err = fmt.Errorf("fileInfo: %w", err)
 	}
 
 	return f, err