Adding more bug checks

I added some checks in the player class at the startExplication function that checks if the src (the url for the audio) is valid or not by doing an fetch with the HEAD method and checking if the return is an valid audio before trying anything in this function. Also i solved this error in the same function:

Uncaught (in promise) AbortError: The play() request was interrupted because the media was removed from the document. https://goo.gl/LdLk22

This error was because the startExplication function where trying to play the audio before it was loaded, i solved it by making it play the audio just after it was load with the event listener canplaythrough.

There was another error at the playMusic function where the code where trying to play the youtube video before it was loaded, i solved it by making it wait until it was CUED (Where the video isnt playing, but is loaded and showed).

(The website was not created to be in mobile and dont has a lot of support, the video where recorded in my smartphone because i had to record the audio and the audio on my pc dont work. In the Future i Will add mobile suport, but right now im finishing the bugs and etc)