A few bug fixes.

- The logic for unterminated comment blocks was inverted.
- Added exception handler for data_map::key_error in NodeFor::gettext()
  so if the for loop list key path doesn't exist the whole template
  won't stop processing.
- Replaced use of std::stoul() with std::strtoul() to fix build failure
  when using gcc STL libs.

Signed-off-by: Chris Reed <flit@me.com>