Compilation errors cause `gulp:watch` and `playground` tasks to fail & exit

If you run the `gulp:watch` or `playground` tasks and make a coding error that causes a compilation failure, it stops the watch task and requires a restart. The failure seems to happen in the rollup tasks as the index.js file doesn't exist after ngc failure:

[0] [11:36:43] Starting 'rollup:fesm'...
[0]
[0] events.js:160
[0] throw er; // Unhandled 'error' event

Since I would imagine this is quite a common occurrence, and it is annoying to have to keep restarting your watch/playground task mnually, it would be nice to handle compilation errors more gracefully.

Start with the gulp:watch and playground task definitions and trace the rollup tasks around the missing index.js after an ngc failure. Reproduce a compilation error, then verify that the watch or playground task remains running and can recover when the code is corrected.

Written by the indexing model from the issue text.