Instead of using a variable is_initialized throw exceptions

[Rezenders]

Instead of using a variable is_initialized throw exceptions.

[chcorbato]

@Rezenders could you please add more information? E.g. link a relevant excerpt of code.

[Rezenders]

The RosReasoner class has a variable called is_initialized that is only being used at the beginning of the RosReasoner main loop and in mros2_reasoner_node to check if there were any problems initializing the node.
For the first use, it is not really necessary since the loop is the last thing created in the __init__. For the second case, I think it would be better to guarantee that if there are any errors in the initialization process, an exception is thrown. I think the only point we should be worried about is reading the ontology files