CASSANDRA-20422: improve the golang download when building source code

[maoling]

• more details in the CASSANDRA-20422

[michaelsembwever]

LGTM, two small nits.

[smiklosovic]

@maoling if this fails because e.g. we are genuinely offline (e.g. in an airplane / train), then this will echo network issue but it will keep executing the rest of the code. But if it was not downloaded, then tar -C ... fails because TMPDIR/GO_TAR does not exist etc. We should fail in a more graceful manner.

[michaelsembwever]

both should fail hard.

if there's no network we can output that offline only works with go manually installed (or if you use -Dant.gen-doc.skip=true)