Improve trap error messages (#831)

* Improve trap error messages

The new trap error message for the issue #828 looks like:

```
thread 'main' panicked at 'a', /proc/self/fd/11:1:13
note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace.
Error: failed to run main module `test.wasm`

Caused by:
    0: failed to invoke `_start`
    1: wasm trap: unreachable, source location: @6cea
       wasm backtrace:
         0: __rust_start_panic
         1: rust_panic
         2: std::panicking::rust_panic_with_hook::h57f0cff11449798f
         3: std::panicking::begin_panic::hd620695467c5dd1f
         4: test::main::ha54db001eabbde1b
         5: std::rt::lang_start::{{closure}}::h5acfb82693695869
         6: std::sys_common::backtrace::__rust_begin_short_backtrace::h39e8b9420da241f9
         7: std::panicking::try::do_call::hb7ebfcd70d5f703e
         8: __rust_maybe_catch_panic
         9: std::rt::lang_start_internal::hd5f64f52a5c5315c
         10: std::rt::lang_start::h2a51d79994dd0c4b
         11: __original_main
         12: _start
```

Closes #828

* Tidy up the style of the traps tests

* Add some tests and module names

crates/api/src/trap.rs

@@ -78,7 +78,25 @@ impl fmt::Debug for Trap {
 
 impl fmt::Display for Trap {
     fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
-        self.inner.message.fmt(f)
+        write!(f, "{}", self.inner.message)?;
+        let trace = self.trace();
+        if trace.is_empty() {
+            return Ok(());
+        }
+        writeln!(f, "\nwasm backtrace:")?;
+        for (i, frame) in self.trace().iter().enumerate() {
+            let name = frame.module_name().unwrap_or("<unknown>");
+            write!(f, "  {}: {}!", i, name)?;
+            match frame.func_name() {
+                Some(name) => match rustc_demangle::try_demangle(name) {
+                    Ok(name) => write!(f, "{}", name)?,
+                    Err(_) => write!(f, "{}", name)?,
+                },
+                None => write!(f, "<wasm function {}>", frame.func_index)?,
+            }
+            writeln!(f, "")?;
+        }
+        Ok(())
     }
 }