Fixing possible NoClassDefFoundError when trying to load nonexisting classes

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.