running `dune tools exec <p>` when `p` is not already installed as a dev tool should suggest users run `dune tools install <p>`

[shonfeder]

Currently it seems to give different kinds of unhelpful errors. E.g.,

dune tools exec ocamlmerlin
  Error: Unable to load the lockdir for the default build context.
  Hint: Try running 'dune pkg lock

from https://github.com/ocaml/dune/pull/12972/changes#diff-8b4458edb3206a9686f140bdca86b4f98a3c0da2117f24fe040c5dab244ad9b0R12-R14

or

Error: No rule found for
default/.pkg/ocaml-compiler.5.4.0-0420973694f2d2cc3f4f0eb8d8d4f8e7/target/cookie
(context _private)
-> required by Computing closure for package "opam-publish"
Error: No rule found for
default/.pkg/ocaml-base-compiler.5.4.0-681abf63cf2fad33dbc8b99448e71adf/target/cookie
(context _private)
<etc.>

when I just tried locally.

Instead, it should say something like

dune tools exec merlin
  Error: The tool merlin is not installed
  Hint: Try 'dune tools install merlin'