Better insight on dictionary of traits instantiation

[fcollonval]

In the code, when an instance of traits dictionary is created, if an error occurs in a element construction, the original error message is not used. This makes it more complicated to figure out what went wrong.

https://github.com/ipython/traitlets/blob/32ced29b0ab06914ff92e9bdcd8806536633d701/traitlets/traitlets.py#L2835-L2845

It would be nice to add the catch error message to the newly created element error.

 if key_trait: 
     try: 
         key = key_trait._validate(obj, key) 
     except TraitError as error: 
-         self.element_error(obj, key, key_trait, 'Keys') 
+         self.element_error(obj, key, key_trait, str(error), 'Keys') 
 active_value_trait = per_key_override.get(key, value_trait) 
 if active_value_trait: 
     try: 
         v = active_value_trait._validate(obj, v) 
     except TraitError as error: 
-         self.element_error(obj, v, active_value_trait, 'Values') 
+         self.element_error(obj, v, active_value_trait, str(error), 'Values')