Add AppError class - base class for errors
Checkout last version of src/core/handleErrors
Testing and fixed AppError
Rename src/core/BaseError in src/Core/AppError
Errors response json.
String errors defined as 'UnknownError' type
Error response depends on the 'content-type' header of request
Merge branch 'master' into develop-user-model

Conflicts:
	src/core/handleErrors.js