fix missing import in change message

Transported values in a change must be converted first.
As this is only relevant for the exotic "scaled" and "blob"
datatypes, this was not detected yet.

- add tests
- suppress warning PytestUnhandledThreadExceptionWarning in tests
+ change import_value methods to raise no other exceptions than
  WrongTypeError and RangeError
+ simplify Command.do: as import_value already raises the
  appropriate error, no more try/except is needed

Change-Id: I299e511468dc0fcecff4c20cf8a917da38b70786
Reviewed-on: https://forge.frm2.tum.de/review/c/secop/frappy/+/32743
Tested-by: Jenkins Automated Tests <pedersen+jenkins@frm2.tum.de>
Reviewed-by: Enrico Faulhaber <enrico.faulhaber@frm2.tum.de>
Reviewed-by: Alexander Zaft <a.zaft@fz-juelich.de>
Reviewed-by: Markus Zolliker <markus.zolliker@psi.ch>

test/test_modules.py

@@ -77,6 +77,7 @@ class DummyMultiEvent(threading.Event):
         return trigger
 
 
+@pytest.mark.filterwarnings('ignore')  # ignore PytestUnhandledThreadExceptionWarning
 def test_Communicator():
     o = Communicator('communicator', LoggerStub(), {'description': ''}, ServerStub({}))
     o.earlyInit()
@@ -87,6 +88,7 @@ def test_Communicator():
     assert event.wait(timeout=0.1)
 
 
+@pytest.mark.filterwarnings('ignore')  # ignore PytestUnhandledThreadExceptionWarning
 def test_ModuleMagic():
     class Newclass1(Drivable):
         param1 = Parameter('param1', datatype=BoolType(), default=False)