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.

lib/wasm/src/lib.rs

@@ -42,6 +42,10 @@ extern crate cretonne_codegen;
 extern crate cretonne_frontend;
 extern crate wasmparser;
 
+extern crate failure;
+#[macro_use]
+extern crate failure_derive;
+
 mod code_translator;
 mod environ;
 mod func_translator;