cross-build: upgrade zlib to 1.2.12, show curl errors (#915)

cross-build: upgrade zlib to 1.2.12, show curl errors (#915)

* upgrade zlib to 1.2.12

The package for the version 1.2.11 was removed from upstream,
which lets the build script fail.

* cross-build: show curl error if source download fails

Set the curl options `-f (--fail)` and `-S (--show-error)`,
to show errors when curl fails to download a source package.

curl -sLo- "https://zlib.net/zlib-1.2.11.tar.gz" | tar xz
tar: invalid magic
tar: short read

curl -fSsLo- "https://zlib.net/zlib-1.2.11.tar.gz" | tar xz
curl: (22) The requested URL returned error: 404
tar: invalid magic
tar: short read