Handle 500 error for GraphQL "configureSast" mutation

What does this MR do?

Issue: #246738 (closed)

This MR is for the changes made in !43936 (comment 440278905). According to !43936 (comment 440278905), we need to manually create an MR for 13-4-stable-ee-patch-6, as @gitlab-release-tools-bot can not pick up this change automatically.

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Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• [-] Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• Style guides
• [-] Database guides
• Separation of EE specific content

Availability and Testing

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
• [-] Tested in all supported browsers
• [-] Informed Infrastructure department of a default or new setting change, if applicable per definition of done

Security

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• [-] Label as security and @ mention @gitlab-com/gl-security/appsec
• [-] The MR includes necessary changes to maintain consistency between UI, API, email, or other methods
• [-] Security reports checked/validated by a reviewer from the AppSec team