Separate user facing errors from system errors

Epic: https://source.puri.sm/Librem5/use-cases/issues/1, https://source.puri.sm/Librem5/use-cases/issues/95

The error shown in !5 (comment 1962) is nothing a user can deal with so we should make

• errors understandable for users
• write diagnostics messages to stderr / stdout only (using glibs logging and set G_LOG_DOMAIN)

You might want to resort to structured logging: https://blog.gtk.org/2017/05/04/logging-and-more/ by using G_LOG_USE_STRUCTURED.