Exit with a more severe error code if the program traps. (#1274)

* 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.

tests/wasm/unreachable.wat

@@ -0,0 +1,5 @@
+(module
+    (func (export "_start")
+        unreachable
+    )
+)