obspy.core.read() no return from function when GSE2 chksum is wrong

While trying to read old GSE2 files from the archive, I have experienced problems. I have created a minimal example:

```python
import obspy.core

def example():
    try:
        obspy.core.read("/tmp/example2.GSE2")
    except obspy.gse2.libgse2.ChksumError as e:
        print "Exception caught: ", e
    print "test"

example()
print "not reached"
```

The input file example2.GSE2 can be found at http://www.seismo.ethz.ch/static/roman/example2.GSE2

Executing this example script will raise an obspy.gse2.libgse2.ChksumError which is caught. After this, anything can be done within the function, as an example I used the print "test" statement. However, the function never returns, i.e. print "not reached" will not be reached.

The example input file truly has a corrupted header. However, I believe the program should throw an exception in this case and return normally if the exception is dealt with.