Improve error handling for zip install

- minzip: don't crash with corrupted zips (e.g. 4096 null bytes)
- twinstall: output error when zip doesn't contain update-binary
- twinstall: add strerror(errno) to system-level errors
- twinstall: correct message if /file_contexts could not be extracted
- twinstall: use TWFunc::Wait_For_Child for better waitpid errors
- twinstall: minor code cleanup

Change-Id: I53b156b0ec08755af2742bb71d1523ae38f4a82e