add error check into run_browser()
add error check into run_browser()

Currently, vcs-ann is silently fail if w3m is not installed.
This patch adds error check.

Signed-off-by: KOSAKI Motohiro <70a1d3ef3e17a2bb0f09a1b2e6c86f607ed1d6d9@gmail.com>

akr/vcs-ann