Obscure error message when the collection in a for_each cannot be evaluated

which makes it really hard to debug anything.

Plus it is not clear that incomplete values should cause a hard error. Perhaps we just ignore the collection until the value is complete.

No file or test is named. Start by reproducing a for_each whose collection cannot yet be evaluated, then trace the collection-evaluation error path and compare its current message with related errors. Done requires an agreed behavior for incomplete values and an error that makes the failure easier to diagnose.

Written by the indexing model from the issue text.