Fixing possible NoClassDefFoundError bubbling from SettingsBuilder

In order to handle exceptions correctly, when classes are not found, one
needs to handle ClassNotFoundException as well as NoClassDefFoundError
in order to be sure to have caught every possible case. We did not cater
for the latter in ImmutableSettings yet.

This fix is just executing the same logic for both exceptions instead of
simply bubbling up NoClassDefFoundError.

I still do not like the notion of loading classes via the SettingsBuilder, but that is a another issue :-)