Wrong error message on owners_before not being a list

[Amartya Gaur]

Describe the bug
When we are trying to transfer an asset, the driver/common/transaction.py file throws an error if the "owners_before" is not a list instance it throws an error with the message : TypeError: owners_after must be a list instance. The message should point to the right variable

To Reproduce
Steps to reproduce the behavior:
Try transferring an asset with owners_before being a string rather than a list

Expected behavior
The error message should say: TypeError: owners_before must be a list instance.

Logs or terminal output
If applicable, add add textual content to help explain your problem.
site-packages/bigchaindb_driver/common/transaction.py", line 86, in init
raise TypeError('owners_after must be a list instance')
TypeError: owners_after must be a list instance

Desktop (please complete the following information):

• Distribution: Manjaro
• Python full version: Python 3.6.10