* PkgConfigNotFound             - not installed
* PkgConfigError                - base pkg-config errors
    * PkgConfigModuleNotFound   - pc file for module was not found
    * PkgConfigModuleVersionNotFound - requested version was not found

Boilerplate now looks
```
from cffi.error import PkgConfigNotFound, PkgConfigError
...
try:
    # try pkg-config way
    ffibuilder.set_source(..., pkgconfig=["libczmq >= 4.0.0"])
except PkgConfigNotFound as e:
    # if pkg-config is not installed, try backup
    ffibuilder.set_source(..., libraries=["czmq", "zmq", "uuid", "pgm"])
except PkgConfigError as e:
    # here we catch both PkgConfigModuleNotFound and PkgConfigModuleVersionNotFound
    # and raise it again - simply to show they are raised
    raise e from None
```