Show installation instructions for glance client on SLE12 if not installed

I'm still not fully convinced that your suggestion is more elegant.

We needed to extend the exception class in a way that
- it can distinguish between SLES12 and openSUSE
- it knows the appropriate SLES module needed for the missing package
- it assembles the right text accordingly

That's a lot of logic which is not related to handling an exception.

A cleaner approach is:
The function which raises the exception knows the problem and therefor generartes the message. Right now, all texts for exceptions are declared in the function that raises the exception. We shouldn't handle Machinery::Errors::MissingRequirement exceptions differently just for this single case.