using an unimported type gives a bad error message

[johnynek]

For instance if you import and call a function that returns a type say Prop, but you haven't imported the Prop, you get an unknown type error:

unknown type: Bosatsu/Properties::Prop

but it should say, unimported type, since we know that type exists. We should give an error message like: you must add from Bosatsu/Properties import Prop to this file.

[johnynek]

A short term improvement would be to change the error message here:

bosatsu/core/src/main/scala/org/bykn/bosatsu/PackageError.scala

Line 531 in a472c8d

case Infer.Error.UnknownDefined(const, reg) =>

to say "unimported type" not "unknown". We know the type since we have the full name, but we haven't imported it fully. That is an improvement until #946 is implemented.