Make the trap name for unreachable traps more descriptive. (#3568)

Following up on WebAssembly/wasi-sdk#210, this makes the trap message
for `unreachable` traps more descriptive of what actually caused the
trap, so that it doesn't sound like maybe Wasmtime itself executed a
`unreachable!()` macro in Rust.

Before:
```
wasm trap: unreachable
wasm backtrace:
     [...]
```

After:
```
wasm trap: wasm `unreachable` instruction executed
wasm backtrace:
     [...]
```

crates/wasmtime/src/trap.rs

@@ -119,7 +119,7 @@ impl fmt::Display for TrapCode {
             IntegerOverflow => "integer overflow",
             IntegerDivisionByZero => "integer divide by zero",
             BadConversionToInteger => "invalid conversion to integer",
-            UnreachableCodeReached => "unreachable",
+            UnreachableCodeReached => "wasm `unreachable` instruction executed",
             Interrupt => "interrupt",
         };
         write!(f, "{}", desc)