ClassCastException when Sched returns Error-Response

[giuseppeman]

When the Sched-API throws an error, it returns 200:OK with a String message containing the detailed error "e.g. ERR: wrong email".
The Sched-Client cannot handle this and returns the following error:

class java.lang.String cannot be cast to class software.xdev.sched.model.UserAdded (java.lang.String is in module java.base of loader 'bootstrap'; software.xdev.sched.model.UserAdded is in unnamed module of loader com.amazonaws.services.lambda.runtime.api.client.CustomerClassLoader @1ddc4ec2): java.lang.ClassCastException
java.lang.ClassCastException: class java.lang.String cannot be cast to class software.xdev.sched.model.UserAdded (java.lang.String is in module java.base of loader 'bootstrap'; software.xdev.sched.model.UserAdded is in unnamed module of loader com.amazonaws.services.lambda.runtime.api.client.CustomerClassLoader @1ddc4ec2)
	at software.xdev.sched.api.UserApi.addUser(UserApi.java:158)
	at software.xdev.sched.api.UserApi.addUser(UserApi.java:81)
	...

This should be handled differently, so that i can find out the error from Sched, without having to work around a class-cast exception.

NOTE: When the Sched API returns an error, if it return 200, the String-Message in the body always starts with "ERR:"