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

cache/filecache/filecache_pruner.go

@@ -14,12 +14,12 @@
 package filecache
 
 import (
+	"fmt"
 	"io"
 	"os"
 
 	"github.com/gohugoio/hugo/hugofs"
 
-	"github.com/pkg/errors"
 	"github.com/spf13/afero"
 )
 
@@ -39,7 +39,7 @@ func (c Caches) Prune() (int, error) {
 			if os.IsNotExist(err) {
 				continue
 			}
-			return counter, errors.Wrapf(err, "failed to prune cache %q", k)
+			return counter, fmt.Errorf("failed to prune cache %q: %w", k, err)
 		}
 
 	}