Add JS_ThrowPlainError

It's a helper for doing the following steps:

- Building an Error object
- Attaching a formatted message
- Throwing the object

Fixes: #375

quickjs.c

@@ -188,6 +188,7 @@ typedef enum JSErrorEnum {
     JS_AGGREGATE_ERROR,
 
     JS_NATIVE_ERROR_COUNT, /* number of different NativeError objects */
+    JS_PLAIN_ERROR = JS_NATIVE_ERROR_COUNT
 } JSErrorEnum;
 
 #define JS_MAX_LOCAL_VARS 65535
@@ -6601,8 +6602,11 @@ static JSValue JS_ThrowError2(JSContext *ctx, JSErrorEnum error_num,
     JSValue obj, ret, msg;
 
     vsnprintf(buf, sizeof(buf), fmt, ap);
-    obj = JS_NewObjectProtoClass(ctx, ctx->native_error_proto[error_num],
-                                 JS_CLASS_ERROR);
+    if (error_num == JS_PLAIN_ERROR)
+        obj = JS_NewError(ctx);
+    else
+        obj = JS_NewObjectProtoClass(ctx, ctx->native_error_proto[error_num],
+                                     JS_CLASS_ERROR);
     if (unlikely(JS_IsException(obj))) {
         /* out of memory: throw JS_NULL to avoid recursing */
         obj = JS_NULL;
@@ -6636,6 +6640,17 @@ static JSValue JS_ThrowError(JSContext *ctx, JSErrorEnum error_num,
     return JS_ThrowError2(ctx, error_num, fmt, ap, add_backtrace);
 }
 
+JSValue __attribute__((format(printf, 2, 3))) JS_ThrowPlainError(JSContext *ctx, const char *fmt, ...)
+{
+    JSValue val;
+    va_list ap;
+
+    va_start(ap, fmt);
+    val = JS_ThrowError(ctx, JS_PLAIN_ERROR, fmt, ap);
+    va_end(ap);
+    return val;
+}
+
 JSValue __attribute__((format(printf, 2, 3))) JS_ThrowSyntaxError(JSContext *ctx, const char *fmt, ...)
 {
     JSValue val;

quickjs.h

@@ -580,6 +580,7 @@ JS_EXTERN JSValue JS_GetException(JSContext *ctx);
 JS_EXTERN JS_BOOL JS_IsError(JSContext *ctx, JSValue val);
 JS_EXTERN void JS_ResetUncatchableError(JSContext *ctx);
 JS_EXTERN JSValue JS_NewError(JSContext *ctx);
+JS_EXTERN JSValue __js_printf_like(2, 3) JS_ThrowPlainError(JSContext *ctx, const char *fmt, ...);
 JS_EXTERN JSValue __js_printf_like(2, 3) JS_ThrowSyntaxError(JSContext *ctx, const char *fmt, ...);
 JS_EXTERN JSValue __js_printf_like(2, 3) JS_ThrowTypeError(JSContext *ctx, const char *fmt, ...);
 JS_EXTERN JSValue __js_printf_like(2, 3) JS_ThrowReferenceError(JSContext *ctx, const char *fmt, ...);