auth: a new error code for the case of password auth against no passw…

[mitake]

…ord user

It will be useful for auditing logs and detecting authentication request with password against a no password user, which only accepts CN based authentication.

cc @xiang90 @gyuho @spzala

[mitake]

I'll add a test case for hitting the new path.

[codecov-io]

Codecov Report

Merging #11795 into master will decrease coverage by 0.51%.
The diff coverage is 0.00%.

@@            Coverage Diff             @@
##           master   #11795      +/-   ##
==========================================
- Coverage   66.22%   65.70%   -0.52%     
==========================================
  Files         403      403              
  Lines       36950    36950              
==========================================
- Hits        24469    24277     -192     
- Misses      10993    11155     +162     
- Partials     1488     1518      +30     

Impacted Files	Coverage Δ
auth/store.go	59.55% <0.00%> (+6.60%)	⬆️
auth/options.go	53.75% <0.00%> (-38.75%)	⬇️
client/client.go	56.53% <0.00%> (-27.46%)	⬇️
client/keys.go	66.83% <0.00%> (-24.63%)	⬇️
auth/simple_token.go	65.54% <0.00%> (-23.53%)	⬇️
pkg/transport/timeout_conn.go	80.00% <0.00%> (-20.00%)	⬇️
etcdserver/api/v3rpc/lease.go	74.68% <0.00%> (-7.60%)	⬇️
pkg/netutil/netutil.go	61.47% <0.00%> (-7.38%)	⬇️
clientv3/concurrency/mutex.go	61.64% <0.00%> (-5.48%)	⬇️
clientv3/retry_interceptor.go	64.21% <0.00%> (-4.42%)	⬇️
... and 23 more

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[spzala]

Thanks @mitake lgtm