feat(InjectionHelper): inject() returns the object for chaining

[keturn]

Allows doing things like

Foo x = inject(new Foo(), context);

compared to previously, when inject() was void:

Foo x = new Foo();
inject(x, context);

Won't break any existing code.

Also includes some minor changes to ContextImpl error messages.

[keturn]

This isn't as necessary with #5033 deciding to use createWithConstructorInjection instead of inject.

But it's still a pretty okay change and the Context error message is a nice improvement too.