server: add sql to com_exec/com_fetch dispatch error log (#12174)

[sre-bot]

cherry-pick #12174 to release-3.0

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What problem does this PR solve?

More and more user use prepare stmt, but current dispatch error log we only can see packet binary info, we don't know which sql meet error.

What is changed and how it works?

add sql to error thrown in handleStmtExec and handleStmtFetch

after this PR, we can got

[2019/09/12 15:38:43.715 +08:00] [WARN] [conn.go:669] ["command dispatched failed"] [conn=1] [connInfo="id:1, addr:127.0.0.1:43540 status:2, collation:utf8_general_ci, user:root"] [command=Fetch] [sql="\u0001\u0000\u0000\u0000\u0001\u0000\u0000\u0000"] [err="12121\ngit.luolix.top/pingcap/tidb/server.(*clientConn).handleStmtFetch\n\t/home/robi/code/go/src/github.com/pingcap/tidb/server/conn_stmt.go:239\ngit.luolix.top/pingcap/tidb/server.(*clientConn).dispatch\n\t/home/robi/code/go/src/github.com/pingcap/tidb/server/conn.go:914\ngit.luolix.top/pingcap/tidb/server.(*clientConn).Run\n\t/home/robi/code/go/src/github.com/pingcap/tidb/server/conn.go:653\ngit.luolix.top/pingcap/tidb/server.(*Server).onConn\n\t/home/robi/code/go/src/github.com/pingcap/tidb/server/server.go:438\nruntime.goexit\n\t/home/robi/runtime/go/src/runtime/asm_amd64.s:1337\nselect 1"]

("select 1" now is in last, we can modify pingcap/errors to let be in front, but we need add msg first)

Check List

Tests

• Unit test
• Integration test

Code changes

• wrap error message with sql

Side effects

• N/A

Related changes

• Need to cherry-pick to the release branch

Release note

• server: add sql to com_exec/com_fetch dispatch error log

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This change is 

[sre-bot]

/run-all-tests

[jackysp]

LGTM

[tiancaiamao]

LGTM

[sre-bot]

/run-all-tests

[sre-bot]

@sre-bot merge failed.

[zz-jason]

/merge

[sre-bot]

Your auto merge job has been accepted, waiting for 12227

[sre-bot]

/run-all-tests