Log JavaScript transpile errors instead of crashing the watcher process.

Problem/Motivation

When saving JavaScript which may contain syntax errors Babel will return an error. Currently this error is just thrown causing the process to crash and no additional changes are watched/saved.

[07:27:09] Watching './**/*.es6.js' for changes.
[07:27:19] 'misc/ajax.es6.js' is being processed.
~/drupal/core/scripts/js/compile.js:21
        throw new Error(err);
        ^

Error: SyntaxError: misc/ajax.es6.js: Unexpected token (13:10)
    at babel.transformFile (~/drupal/core/scripts/js/compile.js:21:15)
    at ~/drupal/core/node_modules/babel-core/lib/api/node.js:141:7
    at FSReqWrap.readFileAfterClose [as oncomplete] (fs.js:438:3)
error Command failed with exit code 1.

Proposed resolution

Catch and log the error, don't crash the watch process.

$ node ./scripts/js/babel-es6-watch.js
[07:26:13] Watching './**/*.es6.js' for changes.
[07:26:36] 'misc/ajax.es6.js' is being processed.
[07:26:37] SyntaxError: misc/ajax.es6.js: Unexpected token (13:10)