Add erorr handling for BHI160  sensors.

See merge request astro/rust-card10!8

Introduces an error type for bhi160 related methods and update
potential failing methods to return results. This should ensure that
it is not possible to encounter a panic during sensor interaction.

This will also:

 - add minor cleanups in `bhi160.rs`,
 - update the example to use the `?` operator instead of `unwrap()`.

this repo is now l0dable-only and l0dables can't be debugged?

Readme: Turn references into links.

See merge request astro/rust-card10!7

fix missing ';' and fix stdint.h and stddef.h header search path as
environment variable