Error handling, exceptions

[jgrewe]

We need a good way of error and exception handling

[stoewer]

For getter methods on attributes or related objects there should be a method to check for null values. And calling a getter of a null value should throw an exception.

e.g.
Block::name() can be checked with Block::nameIsNull() or Block::attrIsNull("name") to ensure that no EAttrIsNull exception will be thrown.

[gicmo]

I would rather call them nameIsSet() or hasName() then IsNull.

[jgrewe]

just wrap things up and close this issue:
Policy is that we offer methods that allow to check for e.g. the existence of groups or attributes that should be called before trying to access the elements.
If one tries to access non-existing objects exceptions are thrown...