Improve Create to show the original exception traceback

[orf]

This small pull request includes the original exception traceback in the re-raised TypeError, re: #3503. If there is an error somewhere during the create operation the error message turns from:

...
File "/home/vagrant/venv/local/lib/python2.7/site-packages/rest_framework/serializers.py", line 863, in create
  raise TypeError(msg)

TypeError: Got a `TypeError` when calling `Document.objects.create()`. This may be because you have a writable...

Original exception text was: 'NoneType' is not iterable.

To the much more helpful:

File "/home/vagrant/venv/local/lib/python2.7/site-packages/rest_framework/serializers.py", line 863, in create
    raise TypeError(msg)

TypeError: Got a `TypeError` when calling `Document.objects.create()`.  This may be because you have a writable field on...

Original exception was:
 Traceback (most recent call last):
  File "/home/vagrant/venv/local/lib/python2.7/site-packages/rest_framework/serializers.py", line 846, in create
    instance = ModelClass.objects.create(**validated_data)
  File "/home/vagrant/venv/local/lib/python2.7/site-packages/django/db/models/manager.py", line 127, in manager_method
    return getattr(self.get_queryset(), name)(*args, **kwargs)
  File "/home/vagrant/venv/local/lib/python2.7/site-packages/django/db/models/query.py", line 348, in create
    obj.save(force_insert=True, using=self.db)
  File "/home/x/document/models.py", line 212, in save
    self.document_template.populate_document(self)
  File "/home/x/document_template/models.py", line 82, in populate_document
    for i in some_method_that_might_return_none():
TypeError: 'NoneType' object is not iterable

[xordoquy]

I wonder if we could take advantage of the nested exceptions with Python 3.x

[orf]

Yes, my first thought that using raise x from y would be perfect, however the six docs imply that this is doesn't do anything on Py2. I figured including the traceback in the message was a good substitute.

[jpadilla]

There might be an interesting opportunity in improve exception messages like this in some places to include the original. Similar case in #3490.

[tomchristie]

Yup, looks good!

[tomchristie]

Leaving this open until have time to also address #3490, and also review if there are any other similar cases we could be addressing.

Pull requests addressing this throughout from anyone else also most welcome if you happen to get to it before I do.

[tomchristie]

On second thoughts may as well merge this now.
Pull requests addressing similar issues elsewhere in the codebase very much welcome.

[orf]

Woo! Thank you @tomchristie, you don't know how happy this makes me.

[tomchristie]

😄 Most welcome - appreciate the input!