Print download error details when using concurrency

[Bo98]

Right now, download errors have very limited usefulness when concurrency is enabled (the default):

$ brew fetch -s hello
✘ Formula hello (2.12.2)
Error: Failed to download resource "hello (2.12.2)"
Download failed: https://example.com/nonexistent

The behaviour without the queue was much more useful:

$ HOMEBREW_DOWNLOAD_CONCURRENCY=1 brew fetch -s hello
==> Downloading https://example.com/nonexistent
curl: (56) The requested URL returned error: 404                                

==> Retrying download in 2s... (1 try left)
==> Downloading https://example.com/nonexistent
curl: (56) The requested URL returned error: 404                                

Error: Failed to download resource "hello (2.12.2)"
Download failed: https://example.com/nonexistent

So let's fix up the concurrent version to display things like this:

$ brew fetch -s hello
✘ Formula hello (2.12.2)
Error: Failed to download resource "hello (2.12.2)"
Download failed: https://example.com/nonexistent
curl: (56) The requested URL returned error: 404 

[Copilot]

Pull request overview

Improves the usefulness of fetch/download failures when Homebrew’s download concurrency queue is enabled by preserving relevant underlying error details (e.g., curl stderr) in the surfaced exception message.

Changes:

• Extend CurlDownloadStrategyError to carry additional error “details” appended to the user-facing message.
• Capture ErrorDuringExecution stderr in CurlDownloadStrategy#fetch and include it in the raised CurlDownloadStrategyError.
• Include explicit details for specific failure paths (e.g., insecure redirect detection, Apache mirror JSON parse failure).

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 4 comments.

File	Description
Library/Homebrew/exceptions.rb	Modifies CurlDownloadStrategyError message construction to append error details.
Library/Homebrew/download_strategy.rb	Propagates underlying stderr into CurlDownloadStrategyError to improve concurrent download failure output.

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[p-linnane]

Makes sense. Thanks!