Cabal does not fail on non-existing system build-tools

Since `Cabal >= 2.0` it is possible to list arbitrary executables as `build-tools`. According to the documentation those executables need to be on the `PATH`. However, `cabal` will silently ignore non-existing executables.

https://github.com/hspec/hspec-example/commit/512125d2fb38a523de71e8ea764bdcccb6b5242f demonstrates the issue.

Steps to reproduce:

```
$ cabal --version
cabal-install version 2.2.0.0
compiled using version 2.2.0.1 of the Cabal library
$ git clone -b build-tools-bug https://github.com/hspec/hspec-example/
$ cd hspec-example
$ cabal new-test
```

Expected behavior: `cabal` complains about the missing build tool

Actual behavior: `cabal` runs the test suite