Handle undefined feature flag case in `featureFlagEnabled` query

Issue: `featureFlagEnabled` GraphQL query fails on FOS... (#480888 - closed)

What does this MR do and why?

The GitLab Workflow extension performs the featureFlagEnabled query for an EE only feature flag, duo_workflow. This causes Web IDE feature specs to fail in FOSS_ONLY mode, where ee code is not available, since the Feature.enabled? method called within the query resolver raises an exception when attempting to check the status of a feature flag that doesn't exist.

This MR attempts to resolve this by catching InvalidFeatureFlagError and passing the error message as a GQL error response.

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Returns GQL error if flag is not defined	Otherwise, returns feature flag value:

• We'll also want to make sure the GitLab FOSS pipeline passes