Improve error handling.

[onelson]

A common pattern in rust libraries is to have a single enum with variants to model the different failure modes. In the initial implementation, the failure modes were handled as "pass-through" from the C library as String.

Once the work for #7 has been completed, we should be able to more comprehensively model these different failure modes as variants. This will result in a breaking API change and so this will be deferred until a later release.