Better error message when using remove gulp.watch functionality

[phated]

In the 4.0 branch, we removed the ability to use strings as watch tasks and it causes a cryptic error message TypeError: Arguments to path.resolve must be strings as noted in #999

We need to make this change/error more apparent.

[ddprrt]

Suggestion:

Error: watching %insert-glob-here%: watch task has to be either gulp.parallel, gulp.series or a function

This would achieve the following:

• Giving a hint as to which watcher we refer to
• Pointing to the new API of gulp.series and gulp.parallel here, which might help people moving from 3.* to 4.0

[phated]

👍 would you be able to submit a PR adding that to index.js?

[tkellen]

Error: watching %insert-glob-here%: watch task has to be a function (optionally generated by using gulp.parallel or gulp.series)

[phated]

Fixed in c98b07a - thanks @ddprrt