ref-filter: how to improve the code

Hi everyone,
I am trying to remove cat-file formatting part and reuse same
functionality from ref-filter.
I have a problem that cat-file sometimes needs to continue running
even if the request is broken, while in ref-filter we invoke die() in
many places everywhere during formatting process. I write this email
because I want to discuss how to implement the solution better.

ref-filter has 2 functions which could be interesting for us:
format_ref_array_item() and show_ref_array_item(). I guess it's a good
idea to print everything in show_ref_array_item(), including all
errors. In that case all current users of ref-filter will invoke
show_ref_array_item() (as they did it before), and cat-file could use
format_ref_array_item() and work with the result in its own logic.

I tried to replace all die("...") with `return error("...")` and
finally exit(), but actual problem is that we print "error:..."
instead of "fatal:...", and it looks funny.
The draft of the code is here: https://github.com/telezhnaya/git/commits/p2

One of the easiest solutions is to add strbuf parameter for errors to
all functions that we use during formatting process, fill it in and
print in show_ref_array_item() if necessary. What do you think about
this idea?

Another way is to change the resulting error message, print current
message with "error" prefix and then print something like "fatal:
could not format the output". It is easier but I am not sure that it's
a good idea to change the interface.

If you have another ideas - please share them with me. It is really
important step to make formatting logic more general and easier to
reuse.

Thanks!
Olga