Exception support for EOFError to access the valid parsed contents
I am currently parsing pcap dumps with pcap.ksy input. if dump is partial (the last packet isn't fully captured), i want to still have the correctly parsed packets. 
Currently the parsing is aborted with an exception EOFError.

if pcap.py is having an exception handling in _read(), i can still access the correctly parsed packets. i checked the user guide and couldn't find a way to mention it in ksy file.

Example (proposal) ksy file to handle this exception:

seq:
  - id: hdr
    type: header
  - id: packets
    type: packet
    repeat: eos
    _except: eoferror
    except-print: incomplete packet_

Generated code can be,

```python
def _read(self):
    self.hdr = self._root.Header(self._io, self, self._root)
    try:
        self.packets = []
        i = 0
        while not self._io.is_eof():
            self.packets.append(self._root.Packet(self._io, self, self._root))
            i += 1
    except EOFError:
         print("incomplete packet")
```

It is just an proposal and can be improved if looks good.