python3-mb: show helpful instructions if libdiscid.so is not installed

[zzarne]

If no native libdiscid is not installed, jack throws OSError: libdiscid.so.0: cannot open shared object file: No such file or directory.

Jack should instruct the user to install it, e.g. apt install libdiscid0.

[pimzand]

I am assuming that every major Linux distribution that comes with libdiscid, will offer a python binding too.
This is the case for Fedora which is what I use, and for Debian too (https://packages.debian.org/sid/python3-libdiscid)

Currently, setup.py forces the installation of a python binding only, but it cannot take care of the discid library.

Would it not be easier for everyone to have users install both packages from their distribution repo, making sure
they are in sync? I'm sure distribution maintainers would require it this way, if they were to include jack as a package.

[pimzand]

Do we need to do something else here? I believe the current situation is satisfactory.
Users will either install python3-libdiscid using apt/yum/dnf/whatever, which will also install the c-library,
or they will install discid using pip or setup.py. Both seem to work.

setup.py will no longer install discid if python3-libdiscid is already installed.