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

hugofs/decorators.go

@@ -14,12 +14,11 @@
 package hugofs
 
 import (
+	"fmt"
 	"os"
 	"path/filepath"
 	"strings"
 
-	"github.com/pkg/errors"
-
 	"github.com/spf13/afero"
 )
 
@@ -232,7 +231,7 @@ func (l *baseFileDecoratorFile) Readdir(c int) (ofi []os.FileInfo, err error) {
 		}
 		fi, err = l.fs.decorate(fi, filename)
 		if err != nil {
-			return nil, errors.Wrap(err, "decorate")
+			return nil, fmt.Errorf("decorate: %w", err)
 		}
 		fisp = append(fisp, fi)
 	}