bcachefs: Better helpers for inconsistency errors

An inconsistency error often happens as part of an event with multiple
error messages, and we want to build up one single error message with
proper indenting to produce more readable log messages that don't get
garbled.

Add new helpers that emit messages to a printbuf instead of printing
them directly, next patch will convert to use them.

Signed-off-by: Kent Overstreet <kent.overstreet@linux.dev>