tag: fix output of "tag -n" when errors occur

tag: fix output of "tag -n" when errors occur

When "git tag" is instructed to print lines from annotated
tags via "-n", it first prints the tag name, then attempts
to parse and print the lines of the tag object, and then
finally adds a trailing newline.

If an error occurs, we return early from the function and
never print the newline, screwing up the output for the next
tag. Let's factor the line-printing into its own function so
we can manage the early returns better, and make sure that
we always terminate the line.

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>