Confusing error message when no unit tests are provided

[Andrew Pennebaker]

When I run `dub test` on a project that does not use any 
`unittest { ... }` sections, then I would expect D to either say 
something like "0 tests run", or perhaps complain that no unit 
tests are implemented.

Instead, I see a low-level error like this:

Error: function `D main` conflicts with static import 
dub_test_root.main

Which doesn't really indicate what is going wrong at a level the 
coder would easily understand.