Better balancer upstream errors when can't connect

[jubrad]

Error with upstream server not available when balancer can't connect to environmentd. Periovusly it this returned an SSL EOF, which was not helpful.

*this does not work for issues where environmentd connections are made and later unexpectedly close, for that we still get the EOFs

Motivation

Error with upstream server not available when balancer can't connect to environmentd.  Periovusly it this returned an SSL EOF, which was not helpful.

Tips for reviewer

Checklist

• This PR has adequate test coverage / QA involvement has been duly considered. (trigger-ci for additional test/nightly runs)
• This PR has an associated up-to-date design doc, is a design doc (template), or is sufficiently small to not require a design.
• If this PR evolves an existing $T ⇔ Proto$T mapping (possibly in a backwards-incompatible way), then it is tagged with a T-proto label.
• If this PR will require changes to cloud orchestration or tests, there is a companion cloud PR to account for those changes that is tagged with the release-blocker label (example).
• If this PR includes major user-facing behavior changes, I have pinged the relevant PM to schedule a changelog post.