Save more data about errors that happen during the R&B process
Test for the Install.wheels attribute
Remove dead code from Mapping
Add function util.safe_constraints()
Add tests for util.{execute_command,get_auth_info_from_url}
Change the signature of Mapping.file_requirement(r, dependencies=[])
Tests for Install.load_installer()
Install.load_installer() should forward errors retrieve/build errors

When a package is not available for installation in the `Install.wheels`
attribute, it means that it couldn't be retrieved or built;

In order to show the proper feedback for the user, the list of built
packages is compared to the initial requirement list, if they're not
equal, the errors captured from the Retrieve & Build process are going
to be forwarded to the caller (the CLI tool).
Return the right error code to the OS after running install