`error::MissingCachedScripts` and `error::MiniscriptPsbtError` are not exported

[tnull]

Describe the bug
error::MissingCachedScripts and error::MiniscriptPsbtError are not exported to the user even though exposed through Error variants. While exposing them is easy enough, I wasn't quite sure what the correct docs are for all of the MiniscriptPsbtError variants.

Expected behavior
Both error types should be exposed and well documented, as otherwise it's hard to know how to handle them properly.

Build environment
BDK 0.28.0

Additional context
Might be a pre-requisite if we were to explore implementing a quick-and-dirty async fork of RpcBlockchain (see #998 for more context).