Warn on energy reading error

[krtab]

Since Linux commit 949dd0104c496fa7c14991a23c03c62e44637e71 (and Linux 5.10) access to energy_uj is forbidden for non-priviledged users to prevent cryptographic side-channel attacks.

This is correctly identified in the doc here: https://hubblo-org.github.io/scaphandre-documentation/troubleshooting.html#i-get-a-permission-denied-error-when-i-run-scaphandre-no-matter-what-is-the-exporter but the doc is outdated.

On my machine I get no error on sensors unavailability but all readings are instead 0.

This PR produces a warning on unavailable reading and updates the doc accordingly.