Diagnostic for running Wagi component under Spin executor

[itowlson]

If you accidentally forget the executor line on a Wagi component, you get the enigmatic error "failed to find function export canonical_abi_free". This has happened to several of us cough cough - for some of us cough more than once cough cough cough.

This PR provides an additional diagnostic around the error message - a user viewing the logs at the default level will now see:

09:55 $ spin up 
Serving HTTP on address http://127.0.0.1:3000
2022-04-13T21:55:09.687838Z ERROR spin_http_engine: Error processing request: component is not compatible with Spin executor - should this use the Wagi executor?

Caused by:
    failed to find function export `canonical_abi_free`

Very open to improving the message, or to detecting the condition in a less hideous way!

[radu-matei]

Unless we start returning specific errors (as opposed to anyhow::Errors, which we should explore at some point), I don't see a prettier way to check for this error, so this LGTM.

[lann]

Gross! 👍