Detection of failed services during routing

There is a mechanism how to handle network issue during resending request to a service:

https://github.com/zowe/api-layer/blob/606d2b3bcb9ab2b6d324c8cef52102f76055c7db/gateway-service/src/main/java/org/zowe/apiml/gateway/config/NettyRoutingFilterApiml.java#L85-L94

The aim is detect that service is down. In this case GW returns 503 which is ok. But the condition is too general and could include other communication issue. The issue is then log in exception handler:

https://github.com/zowe/api-layer/blob/606d2b3bcb9ab2b6d324c8cef52102f76055c7db/gateway-service/src/main/java/org/zowe/apiml/gateway/controllers/GatewayExceptionHandler.java#L172-L176

It generates debug message like:

A service is not available at the moment to finish request https://<GW host>:<GW port>/<path>: Service is not available at https://<service host>:<service port>

It ignores the cause of the error. It is not helpful to understand the reason of failure.

The aim of this issue is:

• provide beter debug message
• decide what exception could be logged as 503 and what as 500
  • 503 should be only request where is missing the remote site
  • exception about certificate (PKIX, etc.) shouldn't end with 503

Start with the referenced sections of gateway-service/src/main/java/org/zowe/apiml/gateway/config/NettyRoutingFilterApiml.java and GatewayExceptionHandler.java. Trace the exception cause for failed routing and compare missing remote-site failures with certificate-related failures. Done means the debug message preserves the failure reason and only missing remote sites return 503, while certificate exceptions return 500.