Send error response if member list cannot be retrieved

[brandond]

Proposed Changes

• Move error response generation code into util
• Send error response if member list cannot be retrieved
  Prevents joining nodes from being stuck with bad initial member list if there is a transient failure, or if they try to join themselves

I am assuming there was some reason we generated a dummy list instead of sending an error, but it's done this since the initial embedded etcd code was bulk-added by Darren in #1770 so I don't know why this was supposed to be a good idea.

Types of Changes

bugfix

Verification

• See linked issue
• Try joining a node that is behind an external load-balancer that includes the new node in its target poor. For example, using the following haproxy config, where SERVER1 is the existing node, and SERVER2 is the node attempting to join the cluster:

    global
        maxconn 256
        max-spread-checks 0

    defaults
        mode tcp
        timeout connect 10ms
        timeout client  60s
        timeout server  60s

    frontend listener
        bind *:6443
        default_backend servers

    backend servers
        retries 10
        retry-on conn-failure
        server server1 SERVER1:6443 check downinter 5ms inter 1s weight 1
        server server2 SERVER2:6443 check downinter 5ms inter 1s weight 100

This haproxy config will cause it to send traffic to the new server as soon as it is up, but before it is actually ready - which will result in it trying to join itself.

Testing

Linked Issues

• Unrecoverable error when joining node attempts to retrieve etcd member list from itself #9661

User-Facing Change

Further Comments

[codecov]

Codecov Report

Attention: Patch coverage is 15.06849% with 62 lines in your changes are missing coverage. Please review.

Project coverage is 45.90%. Comparing base (fe2ca9e) to head (f9e74e9).
Report is 4 commits behind head on master.

Files	Patch %	Lines
pkg/util/apierrors.go	21.95%	30 Missing and 2 partials ⚠️
pkg/etcd/etcd.go	7.14%	13 Missing ⚠️
pkg/server/router.go	9.09%	10 Missing ⚠️
pkg/server/auth.go	0.00%	4 Missing ⚠️
pkg/daemons/control/tunnel.go	0.00%	3 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #9722      +/-   ##
==========================================
- Coverage   53.04%   45.90%   -7.14%     
==========================================
  Files         154      155       +1     
  Lines       13599    13588      -11     
==========================================
- Hits         7214     6238     -976     
- Misses       5020     6145    +1125     
+ Partials     1365     1205     -160     

Flag	Coverage Δ
e2etests	37.55% <1.36%> (-11.90%)	⬇️
inttests	39.44% <15.06%> (+0.08%)	⬆️
unittests	16.14% <1.81%> (-0.05%)	⬇️

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