Add exception tests for Plug.Builder

The existing tests in` Plug.BuilderTest` don't cover error cases when exception are raised by bad plugs (both compile-time and runtime exceptions). Exception are thrown when:

* a module using `Plug.Builder` never calls the `Plug.Builder.plug/2` macro
* a module plugged through the `Plug.Builder.plug/2` macro doesn't define a `call/2` function
* a module plugged through the `Plug.Builder.plug/2` macro doesn't return a `Plug.Conn` when its `call/2` function is called

I added tests to cover all these cases.