allow to handle error loading and loaded script

[DorianMaliszewski]

Here is a workaround to handle when gtm throw error when loading script and handle when it's loaded.
What do you think about it ?

SImply handle it with window.addEventListener("gtm_loading_error", func) and window.addEventListener("gtm_loaded", func)

[ishahadathb]

@alinemorelli Can we have this PR merged? I also need these changes because sometimes it can fails silently and that causes missing analytics data