improve error handling

in the current implementation, in case polling results in an error,
an error_update is generated. But on activate, for this parameter
a normal update will be generated, indicating that this value is ok.

With this change, parameters have an additonal attribue 'readerror',
which save the error generated on the last read, or None, if the
read was successful.

In addition, subsequent repeated errors are only reported the first
time.

Change-Id: I39f18d17debadd8aa5b904e59998d2172f3f11b9
Reviewed-on: https://forge.frm2.tum.de/review/c/sine2020/secop/playground/+/21974
Tested-by: JenkinsCodeReview <bjoern_pedersen@frm2.tum.de>
Reviewed-by: Markus Zolliker <markus.zolliker@psi.ch>

test/test_poller.py

@@ -157,6 +157,9 @@ class Module:
     def isBusy(self):
         return self.is_busy
 
+    def pollOne(self, pname):
+        getattr(self, 'read_' + pname)()
+
     def __repr__(self):
         rdict = self.__dict__.copy()
         rdict.pop('parameters')