cli: propagate batch tagging warnings to exit value

cli: propagate batch tagging warnings to exit value

In case last input for batch tagging was either invalid or skippable
line, notmuch command exited with non-zero value.
After this change if there is at least one invalid line, notmuch
command will exit with non-zero value. Additionally, skipped lines
(last or other) doesn't cause non-zero value to be returned.