cannot build when loaf not compiled
When the `loaf` binary is not compiled, the error given by the compiler is

```
stack trace: (most recent call last)
/home/user/nimterop/nimterop/build/getheader.nim(485, 23)
/home/user/nimterop/nimterop/build/getheader.nim(234, 11) buildLibrary
/home/user/nimterop/nimterop/build/tools.nim(198, 11) make
/home/user/nim-1.2.6/lib/system/assertions.nim(29, 26) failedAssertImpl
/home/user/nim-1.2.6/lib/system/assertions.nim(22, 11) raiseAssert
/home/user/nim-1.2.6/lib/system/fatal.nim(49, 5) sysFatal
/home/user/liblibrary-to-build/src/nimsignal.nim(19, 10) template/generic instantiation of `getHeader` from here
/home/user/nim-1.2.6/lib/system/fatal.nim(49, 5) Error: unhandled exception: /home/user/nimterop/nimterop/build/tools.nim(198, 12) `findFile(check, path, regex = regex).len != 0` make failed [AssertionError]
```

which is relatively unhelpful, compared to the error message given [when the compiled `toast` binary does not exist](https://github.com/nimterop/nimterop/blob/e61dc54a898a395902205b5794e09ece0e9b35ba/nimterop/cimport.nim#L45).