fix(nodeenv): report a missing node archive instead of a traceback

[ekalinin]

Closes #250

Problem

A 404 on the node.js archive ended in a raw urllib traceback:

 * Install prebuilt node (99.99.99) ..
Traceback (most recent call last):
  ...
urllib.error.HTTPError: HTTP Error 404: Not Found

Worse, on arm64 the url was never named at all. The x64 retry ran inside
the except urllib2.HTTPError block, so a second 404 was raised "during
handling of the above exception" (hence the doubled traceback in the issue),
and logger.warning('Failed to download from %s') was never reached.

Fix

• Build the list of urls to try up front, so the x64 retry happens outside the
  handler and failures are no longer chained.
• Report every attempted url with its http status, and add a hint about
  --list / --source when the archive is simply not there.

Before:

 * Install prebuilt node (99.99.99) ..
Traceback (most recent call last):
  ... 40 lines ...
urllib.error.HTTPError: HTTP Error 404: Not Found

After:

 * Install prebuilt node (99.99.99) ..
Error: cannot download https://nodejs.org/download/release/v99.99.99/node-v99.99.99-darwin-arm64.tar.gz: HTTP 404 Not Found
Error: cannot download https://nodejs.org/download/release/v99.99.99/node-v99.99.99-darwin-x64.tar.gz: HTTP 404 Not Found
Error: there is no prebuilt node.js 99.99.99 for darwin-arm64
Error: check "nodeenv --list" for the available versions, or build from source with --source

A non-404 status (500, 403) is reported with its url and status, without the
"try another version" advice.

Tests

Four new tests cover the prebuilt 404, the arm64 fallback that also 404s, the
--source 404 and a 500. Five existing tests asserted pytest.raises(HTTPError)
and were updated to SystemExit; their assertions sat inside the raises block
and never ran, so they were moved out.

Manually verified on darwin-arm64: 22.11.0 installs, 14.21.3 still falls back to
x64, 99.99.99 fails with the message above.