Rework download failures for better error reporting

[Keryan-dev]

Early WIP for eventual discussions.

General wishlist so far:

• Better early error diagnostic
• Reduce generic error format uses to a minimum
• Eliminate raw exception printing in error messages
• Eliminate assumptions about error string content

[Keryan-dev]

Trimmed up, and undrafted.

Some tests changes, still not ideal but there should be no loss of information.

[leviroth]

I found a case where this change makes the error considerably less descriptive:

# On opam 2.2.1
$ opam repository add example http://example.com
[ERROR] Could not update repository "example": OpamDownload.Download_fail(_, "curl: code 404 while downloading http://example.com/index.tar.gz")
[ERROR] Initial repository fetch failed

# On this branch
$ dune exec src/client/opamMain.exe -- repository add example http://example.com
[ERROR] Could not update repository "example": OpamDownload.Download_fail(_)
[ERROR] Initial repository fetch failed

I believe the issue arises because of the use of Printexc.to_string in various places. As I understand it, the default exception printer is unable to represent variants with arguments. Thus, changing an exception like Download_fail of string option * string into Download_fail of dl_failure means that none of the error description can be rendered by Printexc (as, indeed, the first argument is currently not rendered).