Report log level to stderr

[rpdelaney]

For example:

$ wl-paste > ~/carlsen-kaidanov-2023.pgn
Clipboard content is not available as inferred output type "application/vnd.chess-pgn"
Use "wl-paste --list-types" to view available types. Use "--type" to explicitly specify a type.
$

With this output it's not clear if the file has been created (it has) or if this error is fatal (it's not). Something like this might be better:

$ wl-paste > ~/carlsen-kaidanov-2023.pgn
Warning: 
Clipboard content is not available as inferred output type "application/vnd.chess-pgn"
Use "wl-paste --list-types" to view available types. Use "--type" to explicitly specify a type.
$