converge exit code on a failed run

[stevendborrelli]

If a task fails converge will exit with a 0 code. I think we should exit non-zero if any task fails.

example:

task "check" {
  check = "[[ -f  /bad ]]"
  apply = "bad-command"
}

$ converge apply --local test.hcl 
...
Summary: 2 errors, 1 changes

 * root/task.check: root/task.check still has changes after apply
 * root: error in dependency "root/task.check"

$ echo $?
0