charger: minui error handling fixups

Log an error if minui init fails, and avoid subsequent calls to minui
in this case (which are likely to crash).

Warn if no system text font available.  This is now expected on most
devices, and the fallback text is not expected to be needed, but just
in case.

Avoid the attempt to print text if no system font available, log
warnings instead.

Bug: 80249440
Test: manual: charger mode with no system font
Change-Id: Ib0c761d37e41a893b2a192438eebcf48c4827049