fix error typo
{copy/delete}-directory/files: raise "not dir or file" as exn:fail:filesystem

When a bad path is encountered, the problem should count as a filesystem
exception so that it can be caught with other filesystem exceptions.
raco pkg install: work around un-deletable directories

When installing a package "P" and the usual directory already exists
and cannot be deleted, then use the path "P+1", etc., and record the
alternate path in the package database.