[FIX] odoo: bad imports in tests don't fail silently anymore

Before this fix, you can break a test class by simply adding an
incorrect import like this:
`from gloubiboulga import Casimir`
--> No error message, the test is simply not run

This is due to the fact that we want to ignore ImportError's... only
when there is no `.tests` submodules (actually, we ignored them all!)

This commit fixes the condition and re-enables error logging when tests
actually encounter ImportErrors