Breaking build step should stop execution

[jerger]

If one build step breaks it should stop the build execution. At least for shell this is broken:

Machfile.edn

{breaking
  {update! #$ ["exit" 1]}

 non-breaking
  {update! #$ ["exit" 0]}

 dependand
  {depends [breaking non-breaking]
   update! #$ ["echo" "should not be reached"]}}

mach dependand shows

$ exit 0
$ exit 1
$ echo should not be reached
should not be reached

Example can be found here: https://github.com/DomainDrivenArchitecture/dda-cloudspec/tree/master/test/mach/break_on_exit