Add new errors

lib/errors.js

@@ -1,6 +1,7 @@
 let Errors = {
 	unknown: 'An unknown error occured on our end. Please try again later',
 	accountAlreadyCreated: 'This account has already been created',
+	categoryAlreadyCreated: 'This category has already been created',
 	invalidLoginCredentials: 'Invalid login credentials were provided',
 	requestNotAuthorized: 'The request was not authorized',
 	invalidToken: 'The token provided was not valid',