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.
Remove a stray space. Clean up awkware if result...result construct.
auto merge of #917 : larsbergstrom/servo/task_failure, r=metajack

Pipeline construction now creates a task that supervises all of the child tasks created by script, render, and layout. If any of those fail, it sends a message back to the constellation, which re-creates the pipeline and threads it into the appropriate spot in either the FrameTree or set of pending frame updates.

Also, turn on debug_info for symbols in the Makefile.