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

tpl/transform/unmarshal.go

@@ -14,6 +14,7 @@
 package transform
 
 import (
+	"fmt"
 	"io/ioutil"
 	"strings"
 
@@ -23,9 +24,10 @@ import (
 
 	"github.com/mitchellh/mapstructure"
 
+	"errors"
+
 	"github.com/gohugoio/hugo/helpers"
 	"github.com/gohugoio/hugo/parser/metadecoders"
-	"github.com/pkg/errors"
 
 	"github.com/spf13/cast"
 )
@@ -54,7 +56,7 @@ func (ns *Namespace) Unmarshal(args ...any) (any, error) {
 		data = args[1]
 		decoder, err = decodeDecoder(m)
 		if err != nil {
-			return nil, errors.WithMessage(err, "failed to decode options")
+			return nil, fmt.Errorf("failed to decode options: %w", err)
 		}
 	}
 
@@ -72,7 +74,7 @@ func (ns *Namespace) Unmarshal(args ...any) (any, error) {
 		return ns.cache.GetOrCreate(key, func() (any, error) {
 			f := metadecoders.FormatFromMediaType(r.MediaType())
 			if f == "" {
-				return nil, errors.Errorf("MIME %q not supported", r.MediaType())
+				return nil, fmt.Errorf("MIME %q not supported", r.MediaType())
 			}
 
 			reader, err := r.ReadSeekCloser()
@@ -92,7 +94,7 @@ func (ns *Namespace) Unmarshal(args ...any) (any, error) {
 
 	dataStr, err := types.ToStringE(data)
 	if err != nil {
-		return nil, errors.Errorf("type %T not supported", data)
+		return nil, fmt.Errorf("type %T not supported", data)
 	}
 
 	if dataStr == "" {
@@ -160,7 +162,7 @@ func stringToRune(v any) (rune, error) {
 		if i == 0 {
 			r = rr
 		} else {
-			return 0, errors.Errorf("invalid character: %q", v)
+			return 0, fmt.Errorf("invalid character: %q", v)
 		}
 	}