Enhancement: Improve handling of ErrDot errors in rclone and sftp backends

Since Go 1.19, restic can no longer implicitly run relative executables which
are found in the current directory (e.g. `rclone` if it's found in `.`). This
is a security feature of Go to prevent against running unintended and possibly
harmful executables.

The error message for this was just "cannot run executable found relative to
current directory". This has now been improved to yield a more specific error
message, informing the user how to explicitly allow running the executable
using the `-o rclone.program` and `-o sftp.command` extended options with `./`.

https://github.com/restic/restic/issues/3932
https://pkg.go.dev/os/exec#hdr-Executables_in_the_current_directory
https://go.dev/blog/path-security
