* Problem: API returns 404 for existing empty blocks
Solution: Empty blocks are not store in MongoDB because Tendermint does
not notify BigchainDB about them. In case a user requests a block that
is not in our MongoDB, we check if the requested height is less than or
equal to latest_block_height. If that's the case, we return an empty
block. If the requested height is greater than latest_block_height, then
we return 404.
* Address Lev's comment
* address Lev and Vansh comment
* Problem: Amount error is not tested on the HTTP level.
Solution: A failed web test to reproduce the problem.
* Problem: Invalid transaction posted to Tendermint
Solution: Pass the exception to HTTP POST handler function
* Problem: DoubleSpend and CriticalDoubleSpend not differentiated
Solution: Handle these exceptions differently in `get_spent`
* Problem: No test for checking exception DoubleSpend and CriticalDoubleSpend
Solution: Add necessary tests
* Problem: find doesn't raise IndexError
Solution: Remove exception handling for IndexError
