Return a WasmResult from ModuleEnvironment methods (#886)

* [wasm] return a WasmResult from `declare_table_elements`

This method in particular needs to accommodate failure because any table index other than zero is
currently invalid.

* [wasm] additional failure handling improvements

- Adds `WasmResult<()>` as the return type for most of the `ModuleEnvironment` methods that
previously returned nothing.

- Replaces some panics with `WasmError::Unsupported` now that the methods can return a result.

- Adds a `wasm_unsupported!()` macro for early returns with a formatted unsupported message.

cranelift/wasm/src/environ/mod.rs

@@ -1,6 +1,7 @@
 //! Support for configurable wasm translation.
 
 mod dummy;
+#[macro_use]
 mod spec;
 
 pub use crate::environ::dummy::DummyEnvironment;