Setting a ListSelector to a non-iterable value should be an error

[ceball]

See #212 (#212 (comment))

[tonyfast]

There should be errors. future @jbednar agrees with past @jbednar

[jbednar]

Looks like it is already an error, though maybe we could have a better error message (catching it explicitly):

import param

class S(param.Parameterized):
    ls = param.ListSelector([3], objects=[1,2,3,4])
    
s = S()
s.ls=2

---------------------------------------------------------------------------
TypeError                                 Traceback (most recent call last)
<ipython-input-15-f6e98be75645> in <module>
      5 
      6 s = S()
----> 7 s.ls=2

~/param/param/parameterized.py in _f(self, obj, val)
    316             instance_param.__set__(obj, val)
    317             return
--> 318         return f(self, obj, val)
    319     return _f
    320 

~/param/param/parameterized.py in __set__(self, obj, val)
    938             val = self.set_hook(obj,val)
    939 
--> 940         self._validate(val)
    941 
    942         _old = NotImplemented

~/param/param/__init__.py in _validate(self, val)
   1853 
   1854 class MultiFileSelector(ListSelector):
-> 1855     """
   1856     Given a path glob, allows multiple files to be selected from the list of matches.
   1857     """

TypeError: 'int' object is not iterable