chore: update error message for duplicate organization members (#17594)

Closes coder/internal#345

Author: aslilac

coderd/members.go

@@ -62,7 +62,8 @@ func (api *API) postOrganizationMember(rw http.ResponseWriter, r *http.Request)
 	}
 	if database.IsUniqueViolation(err, database.UniqueOrganizationMembersPkey) {
 		httpapi.Write(ctx, rw, http.StatusBadRequest, codersdk.Response{
-			Message: "Organization member already exists in this organization",
+			Message: "User is already an organization member",
+			Detail:  fmt.Sprintf("%s is already a member of %s", user.Username, organization.DisplayName),
 		})
 		return
 	}

coderd/members_test.go

@@ -26,7 +26,7 @@ func TestAddMember(t *testing.T) {
 		// Add user to org, even though they already exist
 		// nolint:gocritic // must be an owner to see the user
 		_, err := owner.PostOrganizationMember(ctx, first.OrganizationID, user.Username)
-		require.ErrorContains(t, err, "already exists")
+		require.ErrorContains(t, err, "already an organization member")
 	})
 }