spack build-env: error when deps are not installed (#35533)

Currently we attempt to setup the build environment even when
dependencies are not installed, which typically results in error while
searching for libraries or executables in a dependency's prefix.

With this change, we get a more user friendly error:

```
$ spack build-env perl
==> Error: Not all dependencies of perl are installed, cannot setup build environment:
 -   qpj6dw5  perl@5.36.0%apple-clang@14.0.0+cpanm+open+shared+threads build_system=generic arch=darwin-ventura-m1
 -   jq2plbe      ^berkeley-db@18.1.40%apple-clang@14.0.0+cxx~docs+stl build_system=autotools patches=26090f4,b231fcc arch=darwin-ventura-m1
...
$ echo $?
1
```

0 files changed, 0 insertions, 0 deletions