Edge case: using COMPONENTS_MAKE_PATH incorrectly

``` bash
Stanislaw: Components (KIF-3.3.2)> COMPONENTS_MAKE_PATH=./Components.make/KIF/3.3.2/KIF.make components install KIF
thread '<main>' panicked at 'called `Result::unwrap()` on an `Err` value: Error { repr: Os { code: 20, message: "Not a directory" } }', ../src/libcore/result.rs:738
```
