Improve error display

[oligriffiths]

This is a tracker issue for improving error display.

Currently, if a plugin throws an exception, it's kinda hard to track down what's happened, e.g.

yarn run v1.10.1
$ node $NODE_DEBUG_OPTION $(which broccoli) serve
Serving on http://localhost:4200

Built with error:
Could not resolve './foo' from ../../../../var/folders/g7/70cblrm11nx2g393qmjkkwkc0000gn/T/broccoli-11316hdKMiIAzNeA2/cache-2-rollup_js_transformation/build/app.js
        at Rollup (JS Transformation)
-~- created here: -~-
    at new Plugin (/Users/oli/Projects/broccoli-test/node_modules/broccoli-plugin/index.js:7:31)
    at new Rollup (/Users/oli/Projects/broccoli-test/node_modules/broccoli-rollup/dist/index.js:39:9)
    at Object.<anonymous> (/Users/oli/Projects/broccoli-test/Brocfile.js:18:10)
    at Module._compile (internal/modules/cjs/loader.js:678:30)
    at Object.Module._extensions..js (internal/modules/cjs/loader.js:689:10)
    at Module.load (internal/modules/cjs/loader.js:589:32)
    at tryModuleLoad (internal/modules/cjs/loader.js:528:12)
    at Function.Module._load (internal/modules/cjs/loader.js:520:3)
    at Module.require (internal/modules/cjs/loader.js:626:17)
    at require (internal/modules/cjs/helpers.js:20:18)
-~- (end) -~-

Error: Could not resolve './foo' from ../../../../var/folders/g7/70cblrm11nx2g393qmjkkwkc0000gn/T/broccoli-11316hdKMiIAzNeA2/cache-2-rollup_js_transformation/build/app.js
    at error (/Users/oli/Projects/broccoli-test/node_modules/broccoli-rollup/node_modules/rollup/dist/rollup.js:224:15)
    at /Users/oli/Projects/broccoli-test/node_modules/broccoli-rollup/node_modules/rollup/dist/rollup.js:19454:25

(node:11316) UnhandledPromiseRejectionWarning: Error: Could not resolve './foo' from ../../../../var/folders/g7/70cblrm11nx2g393qmjkkwkc0000gn/T/broccoli-11316hdKMiIAzNeA2/cache-2-rollup_js_transformation/build/app.js
    at error (/Users/oli/Projects/broccoli-test/node_modules/broccoli-rollup/node_modules/rollup/dist/rollup.js:224:15)
    at /Users/oli/Projects/broccoli-test/node_modules/broccoli-rollup/node_modules/rollup/dist/rollup.js:19454:25
(node:11316) UnhandledPromiseRejectionWarning: Unhandled promise rejection. This error originated either by throwing inside of an async function without a catch block, or by rejecting a promise which was not handled with .catch(). (rejection id: 3)
(node:11316) [DEP0018] DeprecationWarning: Unhandled promise rejections are deprecated. In the future, promise rejections that are not handled will terminate the Node.js process with a non-zero exit code.