Bugfix/inadequate son handling

[maennel]

This PR attempts to fix #1438.
It adds a check of the passed SON value.

If the SON was neither a dict or None, the raised error was an AttributeError, independent of the given value. The error was raised, since the (native) value did not provide a get(..) method.

[wojcikstefan]

Thanks for this. In my opinion, InvalidQueryError shouldn't be raised in _from_son, which is used for more than just querying. Is there a better place in the querying portion of the code where we can throw this exception before we even call _from_son?

[maennel]

Hm, that makes sense. The portion of code that allows for invalid "SON-values" to pass is this one, preparing the query.
I suggest to add the check in there, then. What do you think?

[maennel]

@wojcikstefan I've updated the PR. Feel free to have a look at it. Now, _from_son returns a more generic error. On the other side, preparing a query raises an appropriate InvalidQueryError.

[wojcikstefan]

@maennel this looks great! Thanks for taking the time to address my concerns.