[MERGE] Handle missing tools gracefully for diff --using

[Ian Clatworthy]

Ian Clatworthy has voted tweak.
Status is now: Conditionally approved
Comment:
The test name should be spelt correctly, either test_execute_missing or, 
better still, test_executable_missing.

For details, see: 
http://bundlebuggy.aaronbentley.com/request/%3C47752B18.5030304%40utoronto.ca%3E