fix: incorrect message for locking for Quince

[DmytroAlipov]

This is a backport from the master.

Description:

Reproduction steps:

1. Two Staff go to Instructors tab
2. Open responses and choose ORA
3. By each staff choose user answer
4. By first staff click on "Start grading" button
5. By second staff click on "Start grading" button

Then for the second Staff it will be displayed:

The message selection process was incorrect because a 409 was received from the server (as intended).

Now this message is displayed:

[openedx-webhooks]

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[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 100.00%. Comparing base (c644da3) to head (587f999).

Additional details and impacted files

@@                     Coverage Diff                      @@
##           open-release/quince.master      #325   +/-   ##
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  Coverage                      100.00%   100.00%           
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  Files                             110       110           
  Lines                            1079      1079           
  Branches                          159       159           
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  Hits                             1079      1079           

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