Grunt errors need to be handled better

[tomgreenfield]

Subject of the issue/enhancement/features

I've seen during development that there are some areas of the Grunt tasks producing unhandled promise rejections. These cause a preview to hang and never complete.

Further to #3066, errors should abort Grunt and be written to stderr so that these can be fed through to the front end.

Your environment

• Framework 5.12
• Authoring tool 0.11 (unreleased)