bisect: improve error message of 'bisect log' while not bisecting

bisect: improve error message of 'bisect log' while not bisecting

'git bisect log' is implemented by a direct invocation of 'cat
"$GIT_DIR/BISECT_LOG"', without any sanity checks.  Consequently,
running 'git bisect log' while not bisecting leads to an error,
because the bisect logfile doesn't exists.  The accompanying error
message

  cat: /path/to/repo/.git/BISECT_LOG: No such file or directory

is neither very helpful nor very friendly.

Instead of blindly trying to cat the log file, first check whether
there is a bisection going on (i.e. the bisect logfile exists), and
die with a more appropriate error message when not.

Signed-off-by: SZEDER Gábor <szeder@ira.uka.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>