Backend shouldn't throw SQL exception

[dae]

this mapping to an unrelated exception should be done at a higher level

Do you think you could use the github context menu on this comment to peel this to a new issue and specify exactly what the design issue is here?

Originally posted by @mikehardy in #202 (comment)

[dae]

I think it might be a bit neater if the backend always throws BackendException, and the exception mapping was done in RustSupportSQLiteDatabase instead. But probably not high up on the list of priorities.