fsys: cached error variable must be mutable

Fixes build on win32:

fsys.cpp: In member function 'bool ucommon::fsys::is_tty() const':
fsys.cpp:328:11: error: assignment of member 'ucommon::fsys::error' in
datetime: fix const inconsistency

This broke the win32 build as the header prototype didn't match the
implementation.
Merge pull request #34 from tmatth/const_error

Const error