Be more user-friendly when trying to access a non existent data file

When sar and sadf try to access a non existent standard daily data file,
tell the user to check if data collecting is enabled.
By doing so, the user goes to the docs looking for how to enable the
data collection, rather than looking for help on why the software seems
to be broken.

Signed-off-by: Sebastien GODARD <sysstat@users.noreply.github.com>