Cosmetic changes
Add printf format attribute to ty_print_error

Fix a few related errors.
Rename ty_print_error to ty_error

Also rename the following error-related symbols:
 * (enum) ty_error to ty_err
 * ty_mask_error to ty_error_mask
 * ty_redirect_error to ty_error_redirect
Fix ty_delete broken error message
Improve error masking functions

Error masking is now stacked, and managed through ty_error_mask(err)
and ty_error_unmask().
Add install commands to CMakeLists.txt