clientv3: don't halt lease client if there is a lease error

[heyitsanthony]

From reverted API change. Figure out what to do with this when API breakage is OK.

[codecov-io]

Codecov Report

❗ No coverage uploaded for pull request base (master@8990126). Click here to learn what that means.
The diff coverage is 77.58%.

@@            Coverage Diff            @@
##             master    #7866   +/-   ##
=========================================
  Coverage          ?   75.61%           
=========================================
  Files             ?      332           
  Lines             ?    26192           
  Branches          ?        0           
=========================================
  Hits              ?    19806           
  Misses            ?     4961           
  Partials          ?     1425

Impacted Files	Coverage Δ
etcdctl/ctlv3/command/lease_command.go	67.07% <0%> (ø)
proxy/grpcproxy/lease.go	89.42% <100%> (ø)
clientv3/concurrency/session.go	95.12% <100%> (ø)
integration/cluster_proxy.go	100% <100%> (ø)
clientv3/lease.go	93.6% <81.25%> (ø)

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[stale]

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