bcachefs: Clean up error reporting in the startup path

It used to be that error reporting in the startup path was done by
returning strings describing the error, but that turned out to be a
rather silly idea - if there's something we can describe about the
error, just print it right away.

This converts a good chunk of code to returning error codes, as is more
typical style.
Signed-off-by: Kent Overstreet <kent.overstreet@gmail.com>