common/file_util: Make GetCurrentDir() return a std::optional

nullptr was being returned in the error case, which, at a glance may
seem perfectly OK... until you realize that std::string has the
invariant that it may not be constructed from a null pointer. This
means that if this error case was ever hit, then the application would
most likely crash from a thrown exception in std::string's constructor.

Instead, we can change the function to return an optional value,
indicating if a failure occurred.