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

[kit-ty-kate]

apart from the one suggestion, the design looks great

[Keryan-dev]

Trimmed up, and undrafted.

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

[kit-ty-kate]

LGTM as a first step

[kit-ty-kate]

I'd recommend to avoid opens.
They make the codebase harder to maintain, read and reason about.

[kit-ty-kate]

this message feels redundant with the ones above

[Keryan-dev]

In what way ? The lack of information should be the object of a patch for checksum related errors.

[kit-ty-kate]

<><> Processing actions <><><><><><><><><><><><><><><><><><><><><><><><><><><><>
[ERROR] multiple-md5.1/i-am-a-patch: Checksum mismatch for file://${BASEDIR}/p.patch:
          expected md5=hash
          got      md5=hash
[ERROR] Failed to get extra source "i-am-a-patch" of multiple-md5.1: Checksum mismatch

Checksum mismatch

"checksum mismatch" is displayed three times one after the other.

I believe the difference is that both errors above are displayed as the error happens and the last one is displayed when the actions finished.

Since this PR is no longer just an API improvement, i think it would be nicer to make it so that such redundant error messages are squashed into one

[Keryan-dev]

I can't find out why, but it appears that OpamConsole.header_error does not actually display the header like it should in the reftests. This is why some errors seem to appear without context.

[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).