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

common/hugio/copy.go

@@ -14,13 +14,12 @@
 package hugio
 
 import (
+	"fmt"
 	"io"
 	"io/ioutil"
 	"os"
 	"path/filepath"
 
-	"github.com/pkg/errors"
-
 	"github.com/spf13/afero"
 )
 
@@ -60,7 +59,7 @@ func CopyDir(fs afero.Fs, from, to string, shouldCopy func(filename string) bool
 	}
 
 	if !fi.IsDir() {
-		return errors.Errorf("%q is not a directory", from)
+		return fmt.Errorf("%q is not a directory", from)
 	}
 
 	err = fs.MkdirAll(to, 0777) // before umask