The message now gives concrete advice to developers who
experience the error due to running multiple simultaneous builds
via webpack.
Fixes#35000
PR Close#35001
Previously, it was possible for multiple instance of ngcc to be running
at the same time, but this is not supported and can cause confusing and
flakey errors at build time.
Now, only one instance of ngcc can run at a time. If a second instance
tries to execute it fails with an appropriate error message.
See https://github.com/angular/angular/issues/32431#issuecomment-571825781
PR Close#34722
Pete Bacon Darwin