Fix: catch exceptions by reference

[lqhl]

When I type a wrong option in command line, boost::po fails to replace the %canonical_option% tag.
For example, PowerGraph should output

Invalid syntax:
         unrecognised option '--threshold'

Instead, it outputs

Invalid syntax:
         unrecognised option %canonical_option%

The reason is that if we don't catch exceptions by reference, we get 'object slicing' which is explained here.