t/perf/fsmonitor: improve error message if typoing hook name

t/perf/fsmonitor: improve error message if typoing hook name

Previously - it would silently run the perf suite w/o using
fsmonitor - fsmonitor errors are not hard failures.
Now it errors loudly.

GIT_PERF_7519_FSMONITOR="$HOME/rs-git-fsmonitorr"
./p7519-fsmonitor.sh -i -v

fatal: cannot run /home/nipunn/rs-git-fsmonitorr:
No such file or directory
not ok 2 - setup for fsmonitor

Signed-off-by: Nipunn Koorapati <nipunn@dropbox.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>