Improve error message "Cannot compile namespaces when the '--isolatedModules' flag is provided"

TypeScript Version: nightly (2.4.0-dev.20170609)

Code

declare module "foo" {}

Expected behavior:

"Cannot compile ambient module declarations" or "Cannot compile module augmentations".

Actual behavior:

"Cannot compile namespaces"

[mhegazy]

related to #15230

[jakebailey]

Just looking at old issues; this was fixed by #40032 (and this issue was a dupe of the above, effectively).