Remove now defunct error message improving code.

It actually makes stuff worse, as the new BroccoliBuildError stuff handles this correctly.

Prior to this PR, we would end up with strange output. Where different errors would essentially get merged together, resulting in:

The Broccoli Plugin: [BroccoliMergeTrees: TreeMerger (appTestTrees)] failed with:
SyntaxError: dummy/tests/unit/services/service-worker-test.js: "setTimeout" is read-only

an error from broccoli-babel-tranpiler
- its stack is from broccoli-babel-tranpsiler
claims to be from broccoli-merge-trees
- its instantiation stack + message are from the wrapping broccoli-merge-trees

Now the error is:

Build Canceled: Broccoli Builder ran into an error with Babel plugin. 💥
dummy/tests/unit/services/service-worker-test.js: "setTimeout" is read-only
SyntaxError: dummy/tests/unit/services/service-worker-test.js: "setTimeout" is read-only

ember-cli / broccoli-builder

Remove now defunct error message improving code. #21