on_error could benefit from providing more context

[colincasey]

The signature of the error handler for a Buildpack impl is:

fn on_error(&self, error: libcnb::Error<Self::Error>)

When it comes to reporting errors I find myself having to duplicate useful information such as the buildpack name, version, homepage, etc. It would be preferable if the framework could provide a bit more context in the libcnb::Error::BuildpackError variant such as:

• the serialized buildpack descriptor
• the target architecture
• the app directory

[Malax]

Sounds very reasonable to me! :)