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

source/filesystem.go

@@ -14,11 +14,10 @@
 package source
 
 import (
+	"fmt"
 	"path/filepath"
 	"sync"
 
-	"github.com/pkg/errors"
-
 	"github.com/gohugoio/hugo/hugofs"
 )
 
@@ -49,7 +48,7 @@ func (f *Filesystem) Files() ([]File, error) {
 	f.filesInit.Do(func() {
 		err := f.captureFiles()
 		if err != nil {
-			f.filesInitErr = errors.Wrap(err, "capture files")
+			f.filesInitErr = fmt.Errorf("capture files: %w", err)
 		}
 	})
 	return f.files, f.filesInitErr