fix hierarchical mns w.r.t. MPI installed with a dummy toolchain

enhanced version of #983 by @kcgthb (which should have been opened targeting `develop`
That doesn't work, but this does:

```python
error_msg = ("No compiler available in toolchain %s used to install MPI library %s v%s, "
             "which is required by the active module naming scheme %s.") % tup
```