Allow use of user-defined error types in rusqlite migrations.

[nuttycom]

This builds atop #13 to permit user-defined error types in rusqlite migrations. This is useful for migrations that may need to perform more complex operations that can fail in ways that don't fit correctly into rusqlite's error type.

[aschampion]

doctest failing, possibly due to a missing import.

The generic param and that this makes the rusqlite adapter more different from the postgres one aren't great, but I think it's a reasonable workaround since the underlying problem is that there isn't an ergonomic way for migrations or adapters to have application-specific errors as currently written.

I'm fine with merging and releasing as version 0.2 with the expectation I'll probably change error handling later in another breaking version.

[nuttycom]

Updated & rebased to fix the CI failures.

[aschampion]

Thanks @nuttycom! I'll publish 0.2 tomorrow.

[nuttycom]

Thanks @nuttycom! I'll publish 0.2 tomorrow.

Thank you!

[aschampion]

Published, let me know if you encounter any problems.