A non-existing command may not always result in a "FailedToStart" error,
but can also lead to "normal" finish with a non-zero exit code (from the
shell), which means that people whose remote system is missing the "env"
command currently get no feedback at all when pressing the "Fetch"
button. (Independent of this, the exit code should always be checked
anyway.)

Change-Id: I1387897436825c18e7df2d03d5ee3870ff6b70f5
Reviewed-by: Tobias Hunger <tobias.hunger@digia.com>