Toolkit2: Fix old merge

@sven: Check whether this is done or not.

Two changes I did and you unfortunately undid with your merge (should be easy to fix though):

Make sure you are using ValueError instead of JSONDecodeError(ValueError). Otherwise, the code requires python3 version>=3.5.0. This is a typical error message you get:
```
controller.py:215(getSpec):ERROR Could not read specification file '../Euler_ADERDG.exahype':   'module' object has no attribute 'JSONDecodeError'
```
Make sure mexa.py is python3.

I regex-replaced print (".+"\s*(%\s*$.+$)?)$ with print(\1)