refactor: move format_exception() into utils

Signed-off-by: David Aguilar <davvid@gmail.com>
cmds: catch errors when visualizing commits

Show an error message when the configured history browser is not
available.

Closes #197

Reported-by: Taylor Braun-Jones (nocnokneo) <taylor@braun-jones.org>
Signed-off-by: David Aguilar <davvid@gmail.com>