Class with static fields for referring the subpaths of any uploaded zip (contents, descriptor, etc) and also methods for validity checking
ServerReturnMessages class with all return messages of the server in a centralized place. Also: ServerError utility exception
Using new server errors
Making use of UploadZipStructure
Unrelated to this PR - Removing a pending "entityRef" reference
Merge pull request #20 from jtorrente/servererrors

Improved error management
Merge branch 'master' of https://github.com/mdebenito/mokap-backend into fiteredsearch

Conflicts:
	MokapBackend/src/main/java/es/eucm/mokap/backend/controller/search/MokapSearchController.java
Solved some merge conflicts