improve error handling on client connections

- send a heartbeat, if no events for 5 sec.
  an interrupted connection (not closed by the other end) may
  not be detected for a long time when nothing is sent
+ make the error reply on a non SECoPEror more verbose
  e.g. "KeyError('foo')" instead of just "foo"
+ allow cfg file without nodeinterface
+ shorter logger name in HasIodev

Change-Id: I6b1ff23f9bf8c96feb25af44935596437b7d726f
Reviewed-on: https://forge.frm2.tum.de/review/c/sine2020/secop/playground/+/23098
Tested-by: JenkinsCodeReview <bjoern_pedersen@frm2.tum.de>
Reviewed-by: Markus Zolliker <markus.zolliker@psi.ch>

secop/stringio.py

@@ -208,7 +208,7 @@ class HasIodev(Module):
             opts = {'uri': self.uri, 'description': 'communication device for %s' % name,
                     'export': False}
             ioname = name + '_iodev'
-            iodev = self.iodevClass(ioname, self.log.getChild(ioname), opts, srv)
+            iodev = self.iodevClass(ioname, srv.log.getChild(ioname), opts, srv)
             srv.modules[ioname] = iodev
             self.setProperty('iodev', ioname)