Patch fix #1109

[jackkoenig]

This isn't a full fix, but it's an improvement of the status quo we've had for months.

Given something like:

import chisel3._

class MyModule extends RawModule {
  def func(): Unit = {
    val port = IO(Output(UInt(8.W))) //.suggestName("port")
    port := 3.U
  }
  func()
}

object MyMain extends App {
  println(chisel3.Driver.emit(() => new MyModule))
}

Currently, you get the following error:

[error] Exception in thread "main" java.lang.ClassCastException: chisel3.internal.firrtl.Ref cannot be cast to chisel3.internal.firrtl.ModuleIO

Pretty worthless. This PR changes the error to

[info] [error] RawModule.scala:53: Unable to name port UInt<8>(IO <UNNAMED> in MyModule) in MyModule@a67c67e, try making it a public field of the Module in class chisel3.RawModule

It does not pinpoint the error so I'm not going to call this a full fix, but at least it tells you something.

A better solution would come from adding source locators to ports such that we could then report the line. I think this is helpful enough to be worth merging without having to adopt that larger project.

Related issue: #1109

Type of change: bug fix

Impact: no functional change

Development Phase: implementation

Release Notes
Patch #1109 giving a better error message

[jackkoenig]

h/t @ekiwi for reminding me of this lingering issue

[ekiwi]

Cool! Getting an actual error report instead of throwing an exception while trying to error report is an important improvement!