Two small improvements for event.py

- Do not print error fields set to 'None';
- More informative message for already existing
  resource_ids.

The first modification fixes the following behavior:
```python
In [1]: from obspy.core.event import Origin

In [2]: origin = Origin()

In [3]: origin.latitude = 10.1

In [4]: origin.latitude_errors['uncertainty'] = 0.1

In [5]: origin.latitude_errors['other uncertainty'] = None

In [6]: print origin
Origin(latitude=10.1 [other uncertainty=None, uncertainty=0.1])
```
And corrects it to:
```python
In [6]: print origin
Origin(latitude=10.1 [uncertainty=0.1])
```

Warning: The second modification currently makes `obspy.core.tests.test_event.ResourceIdentifierTestCase` fail, since this test compares two `UTCDateTime()` objects which do not have a `resource_id` attribute.

Why are we using `UTCDateTime()` for this test?