If backend return status code 500, redirect to any your url sample.

APIManagement_RedirectToAnyUrl.xml

<policies>
    <inbound>
        <base />
        <set-backend-service id="apim-generated-policy" backend-id="{your backend id}" />
    </inbound>
    <backend>
        <base />
    </backend>
    <outbound>
        <base />
        <choose>
            <when condition="@(context.Response.StatusCode == 500)">
                <set-status code="307" reason="error" />
                <set-header name="Location" exists-action="override">
                    <value>@("{your redirect url}")</value>
                </set-header>
            </when>
        </choose>
    </outbound>
    <on-error>
        <base />
    </on-error>
</policies>