GCC 8 catch value warning

[iandinwoodie]

@kohler I attempted to make a fork of this repository to resolve the issue myself and open a PR, but the fork was public. Neither of us wants that, so I deleted the fork immediately.

Line 375 in pset6/pong61.cc attempts to catch a polymorphic class type by value:

    } catch (std::system_error err) {

This throws a warning due to the -Wcatch-value option (enabled by -Wall at pset6/build/rules.mk:6):

pong61.cc:375:36: warning: catching polymorphic type ‘class std::system_error’ by value [-Wcatch-value=]
         } catch (std::system_error err) {

The warning can be resolved by catching the system error by reference:

    } catch (std::system_error& err) {