Leeway vet doesn't return a non-zero exit code on failures

[mads-hartmann]

Currently leeway vet doesn't return a non-zero exit code when it fails.

From a gitpod-io/gitpod workspace running 0.5.2-d3ff827:

$ leeway vet
ERRO[0008] components/ide/jetbrains/image: EOF          
$ echo $?
0

Here's a screenshot of one of our builds where you see the same behaviour in Werft:

Keep in mind that this specific error will go away with this PR