don't crash on non-existing roles

[rhomeister]

When a non-existing role is used for as :whenever_roles, the variable host on line 5 in whenever.rake can be nil.

To reproduce, define :whenever_roles as:

set :whenever_roles, -> { :dummy }

[benlangfeld]

I'm not sure we really want to silently fail here. We should probably instead raise a more helpful error explaining the issue. Thoughts?

[bragamat]

@rhomeister can you either rebase your PR and address @benlangfeld's comment or close it ?

cc @benlangfeld