- factor out fsck_err_get()
 - if the "bcachefs (%s):" prefix has already been applied, don't
   duplicate it
 - convert to printbufs instead of static char arrays
 - tidy up control flow a bit
 - use bch2_print_string_as_lines(), to avoid messages getting truncated
Signed-off-by: Kent Overstreet <kent.overstreet@linux.dev>