Recent reorganization of Sync has made it clear that we're not very specific (and sometimes can't be) about reporting problems. We ought to fix this. This has both a UI and a backend component. We need to detect crypto problems (including odd NSS errors like "cipher finalize failed"), and so on, and define error codes and strings to present them to the user. See Bug 618340.
We've improved error reporting. If we need more specific improvements, let's open a new bug.