Opaque errors at times returned from the SDKs

We're usually directly forwarding errors that have been created by wit-bindgen from SDK APIs. This can lead to errors that are less than ideal and only offer the minimal amount of information to the user.

For example, calling key_value::Store::open when the spin.toml manifest does not contain a key_value_stores key for the component simply displays the following:

Handler returned an error: Error::AccessDenied

If the user is unfamiliar with the key_value feature, they have absolutely no idea that this error means that they do not have any allowed stores configured. Ideally we would show something like the following:

Handler returned an error: Error::AccessDenied

Note: Are you sure you've included a list of allowed key_value_stores in your spin.toml? 
Make sure to add the `key_value_stores` key to your component and specify which key value stores you'd like to allow. 

For example, `key_value_stores = ["default"]` allows the default key value store.  

Unfortunately, each SDK would have to handle this themselves. The Rust SDK could do this by making the Display implementation more sophisticated than it currently is. However, to make it really nice where we have configurable "verbosity" of error reporting, we'd likely want a bespoke trait that extends std::error::Error and provides additional features.

Start with the Rust SDK's Display implementation and the error returned by key_value::Store::open. Determine how SDK errors should expose the missing key_value_stores configuration and what configurable verbosity or additional error behavior is required; done should provide more useful context than the opaque AccessDenied message.