Exit non-zero on a failed download, and pin fetchez to a version with the auth fix

[mmacferrin]

Two loose ends from #23, which is otherwise resolved between fetchez's
redirect fix and IVERT's .netrc handling (#46, #74).

Pin fetchez to >= 0.6.13

The JSONDecodeError in #23 came from fetchez injecting the Earthdata
credentials into the initial Harmony request headers: the redirect to
Earthdata Login dropped them, Earthdata served its HTML login page, and
parsing that page as JSON blew up. matth-love fixed this in
continuous-dems/fetchez#277 with a redirect-safe requests.Session,
first released in fetchez v0.6.13.

IVERT listed fetchez unpinned, so a fresh install could still resolve a
version with the bug and reproduce the original report.

Exit non-zero when part of a download fails

The last open comment on #23: the error was reported, then "skipping"
logic ran and the program exited 0.

A Harmony request that came back with nothing logged a warning, skipped
that sub-region and moved on, and ivert database download still exited
0 — so a download that retrieved nothing at all looked exactly like a
successful one, and a script driving IVERT had no way to tell.

download_new_granules() now returns a DownloadSummary counting how
each sub-region turned out, and the CLI raises a ClickException when
any part failed.

The distinction that matters: only a Harmony request that returned
nothing counts as a failure. A sub-region Harmony served correctly
that added no new granules — the region holds no ICESat-2 data, or every
granule over it is already in the database — is counted separately and is
still a success. Errors are still counted rather than raised inside the
loop, so one unreachable sub-region does not abandon the rest of the
request; whatever did download is saved, and re-running retries only the
parts still missing.

Partial failure also exits non-zero, since a failed part leaves a hole in
the region that was asked for.

Testing

Exercised through the real CLI path with a stubbed IS2Database:

Summary	Exit	Message
total failure	1	"The download failed. No ICESat-2 data was retrieved."
partial (1 of 3 failed, 5 granules saved)	1	names the counts, says a re-run retries only what's missing
datum mismatch	1	the specific logger.error prints at every verbosity level
clean success	0	—
region with no data	0	—
nothing new to download	0	—

An AST pass over download_new_granules confirms no bare return
remains, so the CLI cannot receive None. prek run --all-files passes.

Follow-up

The conda-forge feedstock recipe still lists fetchez unpinned and needs
the same >= 0.6.13 bound, or a conda install keeps the hole this closes.

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🔍 Docs preview: https://ivert--100.org.readthedocs.build/en/100/