make error message for unknown extension version more useful

[zapashcanon]

Hi,

Here's what happened to me, I wanted to make a library melange-compatible, so I added a:

(modes native melange)

And got the error:

$ dune b @install
File "src/dune", line 4, characters 15-22:
4 |  (modes native melange)
                   ^^^^^^^
Error: 'melange' is available only when melange is enabled in the
dune-project file. You must enable it using (using melange ..) in your
dune-project file.
Note however that the currently selected version of dune (3.0) does not
support this plugin. The first version of this plugin is 0.1 and was
introduced in dune 3.8.

Then, I added the following to my dune-project:

(using melange 1.0)

And here's what happened:

File "dune-project", line 4, characters 15-18:
4 | (using melange 1.0)
                   ^^^
Error: Version 1.0 of the Melange extension is not supported.
There are no supported versions of this extension in version 3.0 of the dune
language.

Both error messages are not helpful enough. I'd like them to list all available versions because I have no idea what they are or how to display them...

(On a related side-note, the first error message is also quite frustrating, it is basically: I understood what you're asking for but I won't do it, it should just offer to add this automatically with whatever version of the extension)