improve error message when task cannot build

[edublancas]

When a user executes a task via Task.build, we verify that the upstream dependencies exist (since those are the inputs for the file the user wants to execute). However, the error message isn't descriptive enough and only tells the user the names of the tasks whose outputs are missing. It'd be better also to print specifically which products are missing.

We could use the ProductEvaluator logic. The easiest way I can think of is to store which products are outdated in an instance variable, although doesn't sound like the cleanest implementation

[rishav-karanjit]

Hi @edublancas, I was wondering if you can give more details on the implementations?