improve error messages

- treat validation errors of the result of write_ and read_ messages
  properly
- add info about the called read_* and write_ methods to the error
  message, in case the error is not raised in the outmost method
- as subsequent errors in poll functions are logged only once, log an
  info when a poll function succeeds again
- remove DiscouragedConversion error

Change-Id: Ib66e001cc95de8225751a1464a92594c369ceb3f
Reviewed-on: https://forge.frm2.tum.de/review/c/secop/frappy/+/30788
Tested-by: Jenkins Automated Tests <pedersen+jenkins@frm2.tum.de>
Reviewed-by: Markus Zolliker <markus.zolliker@psi.ch>

test/test_datatypes.py

@@ -26,7 +26,7 @@
 import pytest
 
 from frappy.datatypes import ArrayOf, BLOBType, BoolType, CommandType, \
-    ConfigError, DataType, DiscouragedConversion, EnumType, FloatRange, \
+    ConfigError, DataType, EnumType, FloatRange, \
     IntRange, ProgrammingError, ScaledInteger, StatusType, StringType, \
     StructOf, TextType, TupleOf, ValueType, get_datatype
 from frappy.errors import BadValueError, RangeError, WrongTypeError
@@ -702,7 +702,7 @@ def test_lazy_validation(dt):
     generalConfig.defaults['lazy_number_validation'] = True
     dt('0')
     generalConfig.defaults['lazy_number_validation'] = False
-    with pytest.raises(DiscouragedConversion):
+    with pytest.raises(WrongTypeError):
         dt('0')