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/page_unwrap.go

@@ -14,7 +14,7 @@
 package hugolib
 
 import (
-	"github.com/pkg/errors"
+	"fmt"
 
 	"github.com/gohugoio/hugo/resources/page"
 )
@@ -36,7 +36,7 @@ func unwrapPage(in any) (page.Page, error) {
 	case nil:
 		return nil, nil
 	default:
-		return nil, errors.Errorf("unwrapPage: %T not supported", in)
+		return nil, fmt.Errorf("unwrapPage: %T not supported", in)
 	}
 }