Objects extending Bundle give incorrect error messages

It's incorrect for an object to extend Bundle, but it gives a truly weird error message:
```scala
object ObjectBundle extends Bundle {
val foo = UInt(8.W)
}

class Example extends Module {
val in = IO(Input(ObjectBundle))
val out = IO(Output(ObjectBundle))
out := in
}
```
Gives:
```
java.lang.AssertionError: assertion failed: The Chisel compiler plugin is now required for compiling Chisel code. Please see https://github.com/chipsalliance/chisel3#build-your-own-chisel-projects.
at ... ()
at Playground$ObjectBundle$.(main.scala:5)
at Playground$ObjectBundle$.(main.scala:5)
at Playground$Example$$anonfun$2$$anonfun$apply$1.apply(main.scala:10)
at Playground$Example$$anonfun$2$$anonfun$apply$1.apply(main.scala:10)
at chisel3.internal.prefix$.apply(prefix.scala:48)
at Playground$Example$$anonfun$2.apply(main.scala:10)
at Playground$Example$$anonfun$2.apply(main.scala)
at chisel3.internal.plugin.package$.autoNameRecursively(package.scala:33)
at Playground$Example.(main.scala:10)
at Playground$$anonfun$5.apply(main.scala:16)
at Playground$$anonfun$5.apply(main.scala:16)
at ... ()
at ... (Stack trace trimmed to user code only. Rerun with --full-stacktrace to see the full stack trace)
```
https://scastie.scala-lang.org/duach2TPTF6eQrZarKCPug

This should be pretty easy to pattern match and report a compile-time error on in the plugin.

**Type of issue**: bug report | feature request

**Impact**: no functional change

**Development Phase**: proposal

**Other information**

**If the current behavior is a bug, please provide the steps to reproduce the problem:**

**What is the current behavior?**

**What is the expected behavior?**

**Please tell us about your environment:**

**What is the use case for changing the behavior?**

Start by reproducing the ObjectBundle example from the issue and inspect the Chisel compiler plugin's handling of objects extending Bundle. The change is complete when this invalid declaration produces a specific compile-time error instead of the misleading compiler-plugin assertion.

Written by the indexing model from the issue text.