[aggregator] Prevent tcp client panic on nil placement

[jcerniauskas]

What this PR does / why we need it:
When TCP Client fails to get placement from KV (f.e. on network partition) on Init() it will return an error, but placementWatcher still does the right thing and tries to reconnect to KV. Before it does so, however, it will return a nil stagedPlacement and any other client methods which read the placement will panic due to NPE.

This PR makes it return an error instead, so client's callers can retry writes on errors and once connection to KV is restored the client will pick up the placement.

Special notes for your reviewer:

Does this PR introduce a user-facing and/or backwards incompatible change?:

NONE

Does this PR require updating code package or user-facing documentation?:

NONE

[CLAassistant]

All committers have signed the CLA.

[codecov]

Codecov Report

Merging #3139 (e108bf7) into master (922a33b) will increase coverage by 1.4%.
The diff coverage is 42.8%.

@@            Coverage Diff            @@
##           master    #3139     +/-   ##
=========================================
+ Coverage    70.8%    72.2%   +1.4%     
=========================================
  Files        1083     1084      +1     
  Lines      100219   100242     +23     
=========================================
+ Hits        71005    72437   +1432     
+ Misses      24139    22753   -1386     
+ Partials     5075     5052     -23     

Flag	Coverage Δ
aggregator	75.8% <42.8%> (+<0.1%)	⬆️
cluster	84.8% <ø> (-0.1%)	⬇️
collector	84.3% <ø> (ø)
dbnode	78.7% <ø> (+3.4%)	⬆️
m3em	74.4% <ø> (ø)
m3ninx	73.1% <ø> (-0.1%)	⬇️
metrics	20.0% <ø> (ø)
msg	73.8% <ø> (-0.4%)	⬇️
query	67.2% <ø> (ø)
x	80.3% <ø> (-0.1%)	⬇️

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