Allow wasm embedders to reject wasm modules with unsupported features. (#345)

Define `WasmError` (and an accompanying `WasmResult`) to represent
errors translating WebAssembly functions. Make `translate_call` and
related functions return `WasmResult`s so that embedders have the
flexibility to reject features they don't support.

Move `InvalidInput` out of `CtonError` and into `WasmError`, where it's
now named `InvalidWebAssembly`, as it's a WebAssembly-specific error
condition. Also extend it to preserve the original error message and
bytecode offset.

cranelift/src/wasm.rs

@@ -105,7 +105,9 @@ fn handle_module(
     }
 
     let mut dummy_environ = DummyEnvironment::with_flags(fisa.flags.clone());
-    translate_module(&data, &mut dummy_environ)?;
+    translate_module(&data, &mut dummy_environ).map_err(
+        |e| e.to_string(),
+    )?;
 
     terminal.fg(term::color::GREEN).unwrap();
     vprintln!(flag_verbose, "ok");