sql: CANCEL QUERY error tweaks, SHOW SESSIONS semicolon removal

[itsbilal]

• Lowercase CANCEL QUERY errors to follow convention.
• Add pgcode to query cancelled error.
• Remove the semicolon at the end of active_queries in SHOW SESSIONS, to make the output match that of the query field in SHOW QUERIES.

[cockroach-teamcity]

This change is 

[knz]

for i, query := range ...{
   if i > 0 {
      WriteString("; ")
  }
   WriteString(sql)
}

[itsbilal]

Done

[CLAassistant]

All committers have signed the CLA.

[knz]

LGTM modulo clean up in test.

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Reviewed 4 of 4 files at r1, 2 of 2 files at r4.
Review status: all files reviewed at latest revision, 1 unresolved discussion.

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pkg/sql/logictest/testdata/logic_test/show_source, line 158 at r4 (raw file):

SELECT node_id,username,application_name,active_queries FROM [SHOW SESSIONS] WHERE active_queries != ''
----
node_id  username  application_name  active_queries                                   

Remove the trailing whitespaces.

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Comments from Reviewable

[itsbilal]

Review status: all files reviewed at latest revision, 1 unresolved discussion.

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pkg/sql/logictest/testdata/logic_test/show_source, line 158 at r4 (raw file):

Previously, knz (kena) wrote…

Remove the trailing whitespaces.

Done.

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Comments from Reviewable