Provide require message for negative widths

core/src/main/scala/chisel3/Width.scala

@@ -36,7 +36,7 @@ sealed case class UnknownWidth() extends Width {
 }
 
 sealed case class KnownWidth(value: Int) extends Width {
-  require(value >= 0)
+  require(value >= 0, s"Widths must be non-negative, got $value")
   def known: Boolean = true
   def get:   Int = value
   def op(that: Width, f: (W, W) => W): Width = that match {

src/test/scala/chiselTests/UIntOps.scala

@@ -602,4 +602,9 @@ class UIntOpsSpec extends ChiselPropSpec with Matchers with Utils with ShiftRigh
       withOptFirrtl should include(line)
     }
   }
+
+  property("UInts with negative widths should have a decent error message") {
+    val e = the[IllegalArgumentException] thrownBy (UInt(-8.W))
+    e.getMessage should include("Widths must be non-negative, got -8")
+  }
 }