Fix for #8777 (continuing PR #9061)

Now filter name is trimmed and lowercased before evaluation for case
insensitive and whitespace trimemd check.

Signed-off-by: Oh Jinkyun <tintypemolly@gmail.com>
@thaJeztah Yeah it does seem weird. How about just "Invalid filter '%s'"?
For formatting, both `%s` and `%q` will do. The only thing matters is escaping. You can check https://gobyexample.com/string-formatting for detailed information.