Better exception for trying to set a unsettable property on wrap

[dannyroberts]

class Foo(jsonobject.JsonObject):
    @property
    def bar(self):
        return 'bar'

In the shell:

>>> Foo.wrap({'bar': 1})
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/Users/droberts/.virtualenvs/cchq/lib/python2.7/site-packages/jsonobject/base.py", line 582, in wrap
    self = cls(obj)
  File "/Users/droberts/.virtualenvs/cchq/lib/python2.7/site-packages/jsonobject/base.py", line 552, in __init__
    self.set_raw_value(key, value)
  File "/Users/droberts/.virtualenvs/cchq/lib/python2.7/site-packages/jsonobject/base.py", line 570, in set_raw_value
    setattr(self, key, wrapped)
  File "/Users/droberts/.virtualenvs/cchq/lib/python2.7/site-packages/jsonobject/base.py", line 654, in __setattr__
    super(JsonObjectBase, self).__setattr__(name, value)
AttributeError: can't set attribute

There's also the nastier case of a settable property that relies on another property, which can create a race condition.