* Exit with a more severe error code if the program traps.
Previously, the wasmtime CLI would return with a regular failure
error code, such as 1 on Unix. However, a program trap indicates a bug
in the program, which can be useful to distinguish from a simple error
status. Check for the trap case, and return an appropriate OS-specific
exit status.
* Use a loop to iterate over the error causes to find Traps.
* Use anyhow's `chain()` iterator.
* For completeness, handle non-Unix and non-Windows platforms too.
* Add a CLI test for a trapping program.
* Replace a manual `.cause` loop with a `.is` call.
* Correct the expected exit status on Windows.
* Use assert_eq/assert_ne so that if these fail, it prints the output.
