Add hint in case system fails

When setting up my local environment, I set up mate with:
EDITOR="/usr/local/bin/mate -w"
as this does not point to a valid path, calling 'system' fails as of 20568999e0a642d5f5f970e1fc7eeb71f5afb7d0 . Unfortunately, it fails silently: no exception, no error output.
To ease debugging this, I suggest to add some more debug output to allow users to see the fail.