fs: Don't complain about failing to read /proc/self/exe with Boost.filesystem

This is intended to silently fail on Unix-like platforms where no procfs
is available, and indeed that's how it works in the non-BFS
implementation of get_exe_dir(). The caller (currently a single function
in game.cpp/wesnoth.cpp) must be able to deal with this situation
in a graceful fashion.