Improve error type with RpcError & internal code sync

[sampajano]

Summary

• Internal code sync (up to Aug 4, 2021 for now..) & relevant improvements. :)
• Updated Closure dependency to 20210808.0.0 (necessary dependency)

Main code changes:

• Improving error typing to use type RpcError in favor of untyped Error (Thanks @TomiBelan! 😃)
  • Also updated StatusCode to be a enum (not full enum support (e.g. RpcError[RpcError.OK] === 'OK') is there but general === comparison should work)
• Removed AbstractClientBase.MethodInfo

Other changes:

• Unifying error behavior in TS and JS echo examples
• Improved jsunit test runner to throw error when no unit test was found

TODO:

• Also set timeout on HTTP request if deadline for grpc call is set #1063 is clobbered now so we need to redo it as a follow-up.

[stanley-cheung]

Yay! Finally we are able to do the sync!