cmds: fix unicode handling when the editor failure message contains unicode

We were converting the returned exception into a bytestring when the
Edit command failed, which results in garbled output.  Use a unicode
string instead.

Closes #266

Reported-by: Ｖ字龍(Vdragon) <pika1021@gmail.com>
Signed-off-by: David Aguilar <davvid@gmail.com>
doc: update v2.0.4 release notes draft

Signed-off-by: David Aguilar <davvid@gmail.com>