string-format: introduce function to format a bt_error

The CLI has a function to format a bt_error nicely.  We will want to use
it in the Python bindings too, so move it to the string-format
convenience library.  Make it return the formatted string instead of
printing directly to stderr.

I did not try to make this function handle memory allocation errors, as:

1. It's not very likely anyway
2. We are likely in the process of handling an error, I don't think it
   would be useful to return an error code.  If something fails in this
   function, I don't know what else remains to do.

Change-Id: Ia540b95bd9e1aca7899e5fbccfe3fba463457e3c
Signed-off-by: Simon Marchi <simon.marchi@efficios.com>
Reviewed-on: https://review.lttng.org/c/babeltrace/+/2433
Tested-by: jenkins <jenkins@lttng.org>