Follow-up use errors instead of errorMessage for Subscriptions::Security::PolicyProjectCreated

The following discussion from !158028 (merged) should be addressed:

• @aturinske started a discussion: (+3 comments)

  @Andyschoenen why did you create the errorMessage field instead of using errors? errorMessage doesn't seem to trigger the error on the frontend?

  apollo: {
      $subscribe: {
        // eslint-disable-next-line @gitlab/vue-no-undef-apollo-properties
        newlyCreatedPolicyProject: {
          query() {
            return getSecurityPolicyProjectSub;
          },
          variables() {
            return { fullPath: this.namespacePath };
          },
          result({ data: { securityPolicyProjectCreated } }) {
            if (!securityPolicyProjectCreated) return;
  
  
            const { errorMessage, project } = securityPolicyProjectCreated;
  
            console.log('securityPolicyProjectCreated', securityPolicyProjectCreated);
            if (errorMessage) {
              throw new Error(errorMessage);
            }
  
            if (project) {
              this.currentAssignedPolicyProject = {
                ...project,
                branch: project?.branch?.rootRef,
              };
            }
          },
          error(e) {
            console.log(e);
            this.setError(e);
            this.setLoadingFlag(false);
          },
          skip() {
            return !this.glFeatures.securityPoliciesProjectBackgroundWorker;
          },
        },
      },
    },

Implementation plan

1. Add errors to the securityPolicyProjectCreated subscription.
2. Mark errorMessage as deprecated.
3. Update frontend to use errors instead of errorMessage.
4. Remove errorMessage.