Add accountDoesNotExist error

lib/errors.js

@@ -2,6 +2,7 @@ let Errors = {
 	unknown: 'An unknown error occured on our end. Please try again later',
 	accountAlreadyCreated: 'This account has already been created',
 	categoryAlreadyExists: 'This category has already been created',
+	accountDoesNotExist: 'This account does not exist',
 	invalidCategory: 'This category does not exist',
 	invalidLoginCredentials: 'Invalid login credentials were provided',
 	requestNotAuthorized: 'The request was not authorized',