gracefully recover from errors in gulp chain

[outofculture]

CSS syntax errors should not require a server restart. Yeah, I know; yet another thing that will be fixed by gulp 4. https://github.com/floatdrop/gulp-plumber offers a solution in the interim. I did not see an obvious means to include plumber in the gulpfile drizzle uses, however, short of rewriting most of the tasks.

[erikjung]

Related: #89

@outofculture You bring up a good point about the Gulp tasks. We may change it so that the tasks are defined within this project and not a separate package.