Ensure mid-build cleanups cancel and cleanup partial work

[stefanpenner]

this doesn't cleanup anything in plugin constructors, but will cleanup tmp directories created by broccoli for the associated plugins.

Future work should likely ensure all plugins considered by a build, and invoke cleanup on them. in 0.18.x though, we only discover plugins as we build the graph.

This is sufficient to fix the bug I set out to fix, but may requires changes in the future to address the above identified issues.

• [x] make work (test in real situations)
• [x] write tests to confirm it works
• [x] POC for upstream: https://github.com/broccolijs/broccoli/pull/312
• [x] 👀

[stefanpenner]

fixed:

• moved SilentError -> deps
• adjusted overly clever return + assignment statement

[stefanpenner]

released as v0.18.1