fixed exception namespace everywhere

src/ExternalSerializer/SerializerList.php

src/Mapper/SimpleMapper.php

src/Mapper/SingleNamespaceMapper.php

src/PayloadMetadata.php

src/PayloadProcessor.php

src/RemovedClassesDeserializer/RemovedClass.php

...RemovedClassesDeserializer/RemovedClassesDeserializer.php

src/SimpleStatefulTrait.php

src/State.php

src/StateBuilder.php

src/docs/README.md

...rnalSerializer/ShouldNotAllowSerializerForInterfaces.phpt

tests/Stateful/Mapper/PrefixMapperTest.phpt

tests/Stateful/Mapper/SimpleMapperTest.phpt

tests/Stateful/Mapper/SingleNamespaceMapperTest.phpt

tests/Stateful/ObjectStorageTest_testClasses.php

tests/Stateful/PayloadProcessorTest.phpt

...emovedClassesDeserializer/RemovedClassesDeserializer.phpt