Add Exception throwing to Android SDK when an error is returned from FFI

[fluffyemily]

Currently, if we receive an error from the FFI inside the Android SDK, we simply print the error and continue. We should throw an exception instead.

This ticket involves - creating set of Mentat specific Exception classes and throwing them when an error is received from Mentat over the FFI.

[ncalexan]

So I definitely agree we should be exposing Mentat errors as Exception instances to Java, but I'd like to pump the brakes a little bit on the "set of Mentat specific Exception classes". The errors coming out of Mentat are fluctuating wildly -- for example, 1/2 of the transactor errors are just strings -- so let's not tie ourselves to any one representation too soon. Could we do the simplest thing first -- a MentatException with a string description -- and then figure out what value we're really trying to provide before we encode a class hierarchy?

[rnewman]

Something I've used before and think might be valuable here: some errors are recoverable and some are not, and some are external and some caused by your code.

An error during a query might be:

• From SQLite (external) because your disk is full (non-recoverable)
• From SQLite because Mentat produced a malformed SQL query (non-recoverable)
• From Mentat because your query is malformed

At the very least, then, I think it's worth making parse errors — EDN, query, transact — separate and visible to the developer. The rest can be bucketed as "DB error".

[fluffyemily]

The three error types we have on iOS right now are:

public enum QueryError: Error {
    case invalidKeyword(message: String)
    case executionFailed(message: String)
}

public struct MentatError: Error {
    let message: String
}

public enum PointerError: Error {
    case pointerConsumed
}

Adding TransactError and ParseError to this set is easy enough I think. This is the set I was going to replicate in the Java library.