Randomized errors for dynamodb mock

[goodmanship]

Add a way to throw errors randomly for ddb read/writes for testing purposes

See #13
The same feature should also be applied for simulating DynamoDB read/write throttles.
In both cases, we can introduce ERROR_PROBABILITY config variables in the range [0,1] which define the probability for the framework to inject throughput exceeded exceptions.

[whummer]

Added in 3043a2f