schema: move 'concurrent schema update' error to constants

[bigbes]

Closes #404, TNTP-3336

[oleg-jukovec]

I suggest to change the commit title too:

const: add 'concurrent schema update' error constant

Up to you.

Please, add a unit-test for the constant that checks that it value is "concurrent schema update". This test will help to fix this behavior (error string).

[oleg-jukovec]

Linter is red.

[oleg-jukovec]

If we have an issue on GitHub, then it is a good idea to provide a link to it. In this case, a link to ticket in Jira is a good idea, but not required.

Closes #TNTP-3336

->

Closes #404, TNTP-3336

or

Closes #404

[oleg-jukovec]

Please, fix the commit/PR message. See above.