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

resources/resource_transformers/tocss/dartsass/transform.go

@@ -120,18 +120,8 @@ func (t *transform) Transform(ctx *resources.ResourceTransformationCtx) error {
 				return m.Offset+len(m.Line) >= start.Offset && strings.Contains(m.Line, context)
 			}
 
-			ferr, ok := herrors.WithFileContextForFile(
-				herrors.NewFileError("scss", -1, -1, start.Column, sassErr),
-				filename,
-				filename,
-				hugofs.Os,
-				offsetMatcher)
+			return herrors.NewFileErrorFromFile(sassErr, filename, filename, hugofs.Os, offsetMatcher)
 
-			if !ok {
-				return sassErr
-			}
-
-			return ferr
 		}
 		return err
 	}