std: Avoid panics in rust_eh_personality

std: Avoid panics in rust_eh_personality

This commit removes a few calls to panic and/or assert in `rust_eh_personality`.
This function definitely can't itself panic (that'd probably segfault or do
something else weird) and I was also noticing that a `pub extern fn foo() {}`
cdylib was abnormally large. Turns out all that size was the panicking machinery
brought in by the personality function!

The change here is to return a `Result` internally so we can bubble up the fatal
error, eventually translating to the appropriate error code for the libunwind
ABI.