some errors which should be checked are ignored

[qw4990]

Bug Report

Please answer these questions before submitting your issue. Thanks!

1. What did you do?
   If possible, provide a recipe for reproducing the error.

Some errors which should be checked are ignored, which may leads some bugs;

These errors are in this report (search key word "assignment to err" in this page);

2. What did you expect to see?

These errors should be checked;

3. What did you see instead?

These errors are ignored;

4. What version of TiDB are you using (tidb-server -V or run select tidb_version(); on TiDB)?

Release Version: v2.1.0-rc.3-354-g062267674
Git Commit Hash: 0622676
Git Branch: ignored_err
UTC Build Time: 2018-12-24 02:36:24
GoVersion: go version go1.11.3 darwin/amd64
Race Enabled: false
TiKV Min Version: 2.1.0-alpha.1-ff3dd160846b7d1aed9079c389fc188f7f5ea13e
Check Table Before Drop: false

[shenli]

@qw4990 This PR will fix some of the ignored errors in TiDB.

[zimulala]

@qw4990
That PR is merged. If there are still problems, you can reopen it.