Return type normalization, and change return errors from imported functions

lib/wasi/src/lib.rs

@@ -17,6 +17,12 @@ pub use self::utils::is_wasi_module;
 
 use wasmer_runtime_core::{func, import::ImportObject, imports};
 
+/// This is returned in the Box<dyn Any> RuntimeError::Error variant.
+/// Use `downcast` or `downcast_ref` to retrieve the `ExitCode`.
+pub struct ExitCode {
+    pub code: syscalls::types::__wasi_exitcode_t,
+}
+
 pub fn generate_import_object(
     args: Vec<Vec<u8>>,
     envs: Vec<Vec<u8>>,