cmds: fix decode error when launching misconfigured editor

We were casting to unicode() where we were previously casting to str()
to fix a unicode decode error.

The right solution is to not convert at all and get the error
message directly from e.strerror instead.

Closes #298

Reported-by: Ｖ字龍(Vdragon) <pika1021@gmail.com>
Signed-off-by: David Aguilar <davvid@gmail.com>