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/func_translator.rs

@@ -5,9 +5,10 @@
 //! WebAssembly module and the runtime environment.
 
 use crate::code_translator::translate_operator;
-use crate::environ::{FuncEnvironment, ReturnMode, WasmError, WasmResult};
+use crate::environ::{FuncEnvironment, ReturnMode, WasmResult};
 use crate::state::{TranslationState, VisibleTranslationState};
 use crate::translation_utils::get_vmctx_value_label;
+use crate::wasm_unsupported;
 use cranelift_codegen::entity::EntityRef;
 use cranelift_codegen::ir::{self, Ebb, InstBuilder, ValueLabel};
 use cranelift_codegen::timing;
@@ -176,7 +177,7 @@ fn declare_locals(
         I64 => builder.ins().iconst(ir::types::I64, 0),
         F32 => builder.ins().f32const(ir::immediates::Ieee32::with_bits(0)),
         F64 => builder.ins().f64const(ir::immediates::Ieee64::with_bits(0)),
-        _ => return Err(WasmError::Unsupported("unsupported local type")),
+        ty => wasm_unsupported!("unsupported local type {:?}", ty),
     };
 
     let ty = builder.func.dfg.value_type(zeroval);