This changes adds an `MempoolError` field to the `ResponseCheckTx`. This will allow clients to understand that their transaction was rejected from the mempool despite passing the ABCI check.
This change also updates the code to make use of early returns to prevent highly nested code blocks. Namely, it returns when the type assertion fails at the beginning of the method, instead of wrapping the entire method in a large if statement. This has a somewhat large effect on the diff as rendered by github.
addresses: #3546
