Add in a warning for a KeepLast depth of 0.

[clalancette]

It really doesn't make much sense to have a KeepLast depth of 0; no data could possibly be stored. Indeed, the underlying DDS implementations don't actually support this. It currently "works" because a KeepLast depth of 0 is assumed to be system default, so the RMW layer typically chooses "1" instead. But this isn't something we should be encouraging users to do, so add a warning.

Signed-off-by: Chris Lalancette [email protected]

I'll note that I could easily be convinced to make this throw an exception instead. That would be a much stronger warning to users not to do this, but it does break existing practice so has some risk of downstream breakage. Happy to discuss it.