Fix handling of panics in ORPC servers

[arthurp]

Previously, catch_unwind did not work in kernel/. It did work in ktests, which was confusing. It turns out that passing -C panic=unwind to rustc is the only way to make it generate correct unwind landing pads to implement catch_unwind. OSDK already forced this when running ktests, but not in other modes.

These changes:

• Add -C panic=unwind in all cases. This is not ideal, but is what we want for ORPC for the moment. We may need to reevaluate this for performance.
• Improve the panic handling and catching code to correctly pass information from the panic site to the catch site where the panic is converted to an Err. This should make error context much better and avoid silent panics within servers.