bcachefs: Make sure allocation failure errors are logged

The previous patch fixed a bug in allocation path error handling, and it
would've been noticed sooner had it been logged properly.

Generally speaking, errors that shouldn't happen in normal operation and
are being returned up the stack should be logged: the write path was
already logging IO errors, but non IO errors were missed.

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