#11 wiki get on directory pages misleads

Repro: f0rge wiki get docs/ -> 'docs/: escapes wiki' (exit 1); f0rge wiki get docs -> 'docs: io error' (exit 1). Both one-line but wrong/vague: trailing slash is not an escape, and a directory read is not a generic io error. Expect: one-line error naming the page, e.g. 'is a directory'. Scope: cmd/wiki.rs check_rel/get.