* Test Bigchain.get_blocks_status_containing_tx()
exception case
* Test Bigchaindb.has_previous_vote()
* Add missing blank lines (pep 8)
* Group imports
* Move ImproperVoteError into .exceptions.py
* Simplify logic
* Simplify formatting
* Imrpove the docstrings a bit
* Move GenesisBlockAlreadyExistsError
into .exceptions.py
* Remove unused import
* Add missing blank line
* Remove extra blank line
* Group imports from same module together
* Add test for Bigchain.transaction_exists()
There was a problem related to the import of the module
`bigchaindb.web.views`.
The module, when imported, inizialises a new `Bigchain` instance, and
this is wrong for testing and because it's a bad practice.
I spent more or less 2h finding out the problem.
