Pipeline create is now wrapped in a task that supervises the tasks
created by script, rendering, and layout. When any of those fail,
the pipeline is removed from the FrameTree and a new one is created
with a failure HTML file.

Additionally, the top-level Makefile is changed to store debug info.
Transitioned to about:failure instead of having the URL in
Constellation.
Fix formatting issues.
Remove code duplication in finding FrameTrees with a given pipeline id.