Better error handling techniques on the render

[miraclx]

The method of handling the render stderr data streams automatically infer critical errors. This might not always be true. As is with  #13, #87, #90, #79, #96. With the current method, terminalizer fails to render from little things such as GTK CSS theme warnings, warnings from failure to find the appmenu-gtk-modules as it did with mine.
Solution: Await the process to complete execution. If a non-zero return code is gotten, reject the render process.
Opening a PR to address this.

[miraclx]

Fixed in #97.
Credits to @maricn (original issue: #96)
@faressoft, please review.