Decode error when launching editor with wrong editor settings

[Vdragon]

Reproduce steps

1. set a not exist text editor
2. LANGUAGE=zh_TW git cola in a Git repository
3. make some changes in the working tree
4. press 'Enter' in the Diff panel textbox

Traceback

Traceback (most recent call last):

  File "/usr/local/share/git-cola/lib/cola/cmds.py", line 1321, in do_cmd
    return cmd.do()

  File "/usr/local/share/git-cola/lib/cola/cmds.py", line 544, in do
    Interaction.critical(N_('Error Editing File'), message, ustr(e))

UnicodeDecodeError: 'ascii' codec can't decode byte 0xe6 in position 10: ordinal not in range(128)

Environment

git-cola

commit 9431eb8