fix retrieve endpoint response code and add testing

[asraa]

Signed-off-by: Asra Ali asraa@google.com

Summary

Fixes #1040
Fixes #1027
Adds some testing for #1041

This change improves response codes returned by erroneous entries on the entries/retrieve endpoint:

• Returns a 404 when entries are not found via EntryUUID. This would originally be 400.
• Returns 400 when entries are malformed (cannot be unmarshalled or canonicalized). Previously this was 500.
• If well-formed entries are not found by hash or by proposed entry, then we now return 404 instead of 400
• If entries are requested by index and are not found, we return 404 instead of 500

Release Note

• fix!: Response codes for entries/retrieve are changed: 404s are now returned for entries not found by hash, proposed entry, or index. 400s are returned for malformed entries. Previously, server returned 500.

Documentation

[priyawadhwa]

nice!!