cmd/compile: graceful handle error in noder LoadPackage

When syntax.Parse returns error, noder.file will be nil. Currently, we
continue accessing it regardlessly and depend on gc.hidePanic to hide
the panic from user.

Instead, we should gracefully handle the error in LoadPackage, then exit
earlier if any error occurred.

Updates #43311

Change-Id: I0a108ef360bd4f0cc9f481071b8967355e1513af
Reviewed-on: https://go-review.googlesource.com/c/go/+/294030


Trust: Cuong Manh Le <cuong.manhle.vn@gmail.com>
Run-TryBot: Cuong Manh Le <cuong.manhle.vn@gmail.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Matthew Dempsky <mdempsky@google.com>