Improve error handling, check for HTTP status codes in GenericCRUD and derivatives

[adityasaky]

The code snippet used in GenericCRUD to send the actual HTTP request was written for a proof-of-concept, and needs to be fleshed out, with better error handling.

This snippet is also used in the Etcd and Grafeas transports, currently.

in-toto-plugin/src/main/java/io/jenkins/plugins/intotorecorder/transport/GenericCRUD.java

Lines 37 to 39 in 9c60581

	/* FIXME: should handle error codes and other situations more appropriately,
	* but this gets the job done for a PoC
	*/