test_parameters: Ignore specific error message

In Python 3, the error message from the decimal module is
less clear than before.
(It's apparently the price to pay for speed -- Python3 uses
libmpdec as its Decimal implementation by default.)

Don't check for the exact error message.

https://fedorahosted.org/freeipa/ticket/5638

Reviewed-By: Jan Cholasta <jcholast@redhat.com>

ipatests/test_ipalib/test_parameters.py

@@ -1441,8 +1441,7 @@ class test_Decimal(ClassChecker):
         param = self.cls('my_number', precision=1)
         e = raises(ConversionError, param, '123456789012345678901234567890')
 
-        assert str(e) == \
-        "invalid 'my_number': quantize result has too many digits for current context"
+        assert str(e).startswith("invalid 'my_number': ")
 
     def test_exponential(self):
         """