Allow build steps to fail

[object88]

Currently, if a pre or post build step fails, the entire build fails, and the process or test will not restart. A step may be written such that an error code is returned improperly (warnings returned as errors, for example). The user should be able to ignore build step errors and allow the build to continue unabated, on a step-by-step basis.

Property (ignore_failure?) could be expressed as a boolean, with a default of false:

"post-build-steps": [{
  "command": "./bin/flakey_script.sh",
  "ignore_failure": true
}]