Allow `test_throws` to take an user-defined exception type


```julia
module L

type Error <: Exception
    msg::String
end
error(s::String) = throw(Error(s))

end

module T
import Base.Test: @test_throws
import L

@test_throws L.Error L.error("error!")

end
```
returns
```
ERROR: L not defined
 in include at boot.jl:244
 in include_from_node1 at loading.jl:128
while loading ~/julia/t.jl, in expression starting on line 14
```