tikvclient: print error logs only if necessary

[hicqu]

What problem does this PR solve?

When TiDB loses a connection to TiKV, it will try to re-establish it if necessary. Only in this case it needs to print error logs. If the connection is closed by TiDB itself, error logs is not necessary.

What is changed and how it works?

Move error log print code to better place.

Check List

Tests
No code.
We are working on adding a test framework for tikvclient. We will add a new check rule for that: if the connArray is closed, all goroutines created by it shoud exit gracefully.

Code changes
No visible changes out of tikvclient.

Close tikv/client-go#19 and pingcap/go-ycsb#97.

[codecov]

Codecov Report

Merging #10349 into master will decrease coverage by 0.0219%.
The diff coverage is 4.1666%.

@@               Coverage Diff               @@
##             master     #10349       +/-   ##
===============================================
- Coverage   77.6897%   77.6677%   -0.022%     
===============================================
  Files           411        411               
  Lines         85436      85455       +19     
===============================================
- Hits          66375      66371        -4     
- Misses        14101      14124       +23     
  Partials       4960       4960

[lysu]

@hicqu plz help add target host info into batchRecvLoop re-create streaming fail and batchRecvLoop error when receive thx :D

[lysu]

LGTM

[tiancaiamao]

LGTM

[tiancaiamao]

/run-all-tests