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_poller.py

@@ -122,6 +122,7 @@ class Mod1(Base, Readable):
         return 0
 
 
+@pytest.mark.filterwarnings('ignore')  # ignore PytestUnhandledThreadExceptionWarning
 @pytest.mark.parametrize(
     'ncycles, pollinterval, slowinterval, mspan, pspan',
     [  # normal case: