web-worker should postMessage sql-errors (instead of throwing)

[kaizhu256]

chrome, edge, (firefox?) do not seem to honor worker.onerror -- and without some kind of error-feedback from sql.js web-workers, they are unsuitable for production use in many browser scenarios.

this [demonstration] patch provides a bug-workaround by postMessaging back errmsg (rather than throwing it).

here's a fully working standalone html-code for patched worker.sql-wasm-debug (with screenshot):

<script>
/* jslint utility2:true */
(async function () {
    "use strict";
    var callbackCount;
    var callbackDict;
    var worker;
    var workerPostMessage;
    callbackCount = 0;
    callbackDict = {};
    worker = new Worker("assets.sqljs.js");
    worker.addEventListener("message", function (msg) {
    /*
     * this function will dispatch received <msg>.data to <callback>
     */
        var callback;
        callback = callbackDict[msg.data.id];
        delete callbackDict[msg.data.id];
        if (callback) {
            callback(msg.data);
        }
    });
    workerPostMessage = function (msg) {
    /*
     * this function will return a promise to postMessage <opt> to worker
     */
        var callback;
        var err;
        // preserve stack-trace outside of [async] Promise for debugging
        err = new Error();
        return new Promise(function (resolve, reject) {
            callback = function (data) {
                // handle errmsg
                if (data.errmsg) {
                    err.message = data.errmsg;
                    reject(err);
                    return;
                }
                // handle data
                resolve(data);
            };
            // save callback in callbackDict
            callbackCount += 1;
            msg.id = callbackCount;
            callbackDict[msg.id] = callback;
            // postMessage with id
            worker.postMessage(msg);
        });
    };
    // test worker data-handling
    console.log(await workerPostMessage({
        action: "exec",
        sql: "SELECT 1"
    }));
    // test worker err-handling
    try {
        await workerPostMessage({
            action: "exec",
            sql: "syntax error"
        });
    } catch (errCaught) {
        console.error(errCaught.stack);
        window.alert(
            "caught web-worker error\n" + errCaught.stack
        );
    }
}());
</script>

[lovasoa]

See #288