Improve error message when assigning from Seq to Vec (#1239)

chiselFrontend/src/main/scala/chisel3/Aggregate.scala

@@ -204,7 +204,7 @@ sealed class Vec[T <: Data] private[chisel3] (gen: => T, val length: Int)
     * @note the length of this Vec must match the length of the input Seq
     */
   def := (that: Seq[T])(implicit sourceInfo: SourceInfo, moduleCompileOptions: CompileOptions): Unit = {
-    require(this.length == that.length)
+    require(this.length == that.length, s"Cannot assign to a Vec of length ${this.length} from a Seq of different length ${that.length}")
     for ((a, b) <- this zip that)
       a := b
   }