fix: lake: warn on fetch cloud release failure

[tydeu]

If Lake fails to download a cloud release, it will now print a warning indicating that it is falling back to a local build. For example:

[0/2] Downloading cloud_test cloud release
[0/2] Building CloudTest
error: > curl -s -f -o [...] -L [...]
error: external command `curl` exited with code 22
warning: fetching cloud release failed; falling back to local build

[leanprover-community-mathlib4-bot]

Mathlib CI status (docs):

• ❗ Std/Mathlib CI will not be attempted unless your PR branches off the nightly-with-mathlib branch. (2024-02-19 15:15:38)
• ❗ Std/Mathlib CI will not be attempted unless your PR branches off the nightly-with-mathlib branch. Try git rebase 3e5695e07e517ff9576d139cbf6e0ed9caf22128 --onto 496a8d578ed3f22dbfe8dce58a855202ccf7be3e. (2024-02-23 17:22:46)