Attempts to store zip files that failed to download

[shoenig]

This isn't a huge problem since the non-null constraint prevents bad data getting in, but we should proactively not try to save zip files which failed to download and/or have a length of 0 bytes.

2019/07/09 07:46:07 TRACE [proxy-client] making zip proxy request to https://proxy.golang.org/github.com/shurcoo%21l/sanitized_anchor_name/@v/1.0.0.zip
2019/07/09 07:46:08 ERROR [proxy-client] bad response (410) body: not found: invalid version "1.0.0"
2019/07/09 07:46:08 INFO  [downloader] downloaded upstream blob of size: 0
2019/07/09 07:46:08 TRACE [store] put module zip (github.com/shurcooL/sanitized_anchor_name @ 1.0.0)
2019/07/09 07:46:08 ERROR [store] failed to write zip for (github.com/shurcooL/sanitized_anchor_name @ 1.0.0), Error 1048: Column 'zip' cannot be null
2019/07/09 07:46:08 ERROR [downloader] failed to save blob to zip store for (github.com/shurcooL/sanitized_anchor_name @ 1.0.0), Error 1048: Column 'zip' cannot be null
2019/07/09 07:46:08 ERROR [bg-worker] failed to download (github.com/shurcooL/sanitized_anchor_name @ 1.0.0), Error 1048: Column 'zip' cannot be null
2019/07/09 07:46:08 TRACE [problems-downloads] setting problem for module (github.com/shurcooL/sanitized_anchor_name @ 1.0.0)