Better Error Message for "Rule not found for"

[sabine]

Desired Behavior

Error message should be clearer in that this either means that the file you're depending on does not exist, or that a rule is missing.

Example

Instead of

File "dune", lines 6-9, characters 0-120:
6 | (rule
7 |   (targets [template.ml](https://template.ml/))
8 |   (deps [template.eml.ml](https://template.eml.ml/))
9 |   (action (run dream_eml %{deps} --workspace %{workspace_root})))
Error: No rule found for [template.eml.ml](https://template.eml.ml/)

dune could report

File "dune", lines 6-9, characters 0-120:
6 | (rule
7 |   (targets [template.ml](https://template.ml/))
8 |   (deps [template.eml.ml](https://template.eml.ml/))
9 |   (action (run dream_eml %{deps} --workspace %{workspace_root})))
Error: Dependency template.eml.ml not found.

You either need to
* add a rule to generate it, or
* provide the file in the correct location ([path where dune is looking for the file]).

[sabine]

Similarly, when there's a dependency referencing a folder without a dune file in it, it would be good if the error message would reflect that the lack of a dune file is a probably cause for the failure, I guess?

[maiste]

For the first message, we could provide a hint. I don't know how much precise we could be when generating the hint.

Similarly, when there's a dependency referencing a folder without a dune file in it, it would be good if the error message would reflect that the lack of a dune file is a probably cause for the failure, I guess?

If you use (include_subdirs qualified) you can have subfolders without dune file in it. I'm not sure if it would be possible to detect this under these conditions.