Informative error messages on catalogue interloper KeyErrors

[edoaltamura]

We could raise errors with a better-documented message when encountering interloper-related errors, such as KeyError: 'interloper'. For instance, version 0.10.0 cannot load interlopers in zoom simulations, but version 0.15.8 supports them - additional info can be printed such as "Interlopers found in <*.properties> do not match expectations. You can try updating velociraptor by running 'pip install velociraptor --upgrade'".

[MatthieuSchaller]

Right, but improving the error message won't help with people running an old version.