1.13 Update Phase 1

Mojang has changed its manifest format for 1.13.
Forge is no longer a universal jar, it requires more hosted files, all of which are generated by the installer.
We can no longer extract the version manifest from forge's jar and have to include it in the distribution.
This commit adds support for launching forge only, mods are currently not supported from the distribution.
Handling of 1.13 launches are subject to change as we move forward.

app/assets/js/assetexec.js

@@ -33,7 +33,7 @@ process.on('message', (msg) => {
                 res.then((v) => {
                     process.send({result: v, context: func})
                 }).catch((err) => {
-                    process.send({result: err.message, context: func})
+                    process.send({result: err.message || err, context: func})
                 })
             } else {
                 process.send({result: res, context: func})