JavaScript: store validation errors in _debug before throwing

This provides the necessary KSC support to reliably indicate validation
errors and assign them to specific fields in the Web IDE object tree
(this will be implemented as part of
kaitai-io/kaitai_struct_webide#156 for easier
debugging).

shared/src/main/scala/io/kaitai/struct/languages/JavaScriptCompiler.scala

@@ -566,7 +566,15 @@ class JavaScriptCompiler(typeProvider: ClassTypeProvider, config: RuntimeConfig)
     val errArgsStr = errArgs.map(translator.translate).mkString(", ")
     out.puts(s"if (!(${translator.translate(checkExpr)})) {")
     out.inc
-    out.puts(s"throw new ${ksErrorName(err)}($errArgsStr);")
+    val errObj = s"new ${ksErrorName(err)}($errArgsStr)"
+    if (attrDebugNeeded(attrId)) {
+      val debugName = attrDebugName(attrId, NoRepeat, true)
+      out.puts(s"var _err = $errObj;")
+      out.puts(s"$debugName.validationError = _err;")
+      out.puts("throw _err;")
+    } else {
+      out.puts(s"throw $errObj;")
+    }
     out.dec
     out.puts("}")
   }