avoid surprising "not found" error when copying to a http remote

avoid surprising "not found" error when copying to a http remote

git-annex copy --to a http remote will of course fail, as that's not
supported. But git-annex copy first checks if the content is already
present in the remote, and that threw a "not found".

Looks to me like other remotes that use Url.checkBoth in their checkPresent
do just return false when it fails. And Url.checkBoth does display
errors when unusual errors occur. So I'm pretty sure removing this error
message is ok.

Sponsored-by: Jarkko Kniivilä on Patreon