Add possibility to set a fallback url

[svenjungnickel]

Short description of what this resolves:

This commit allows to set a fallback url in case the provided url is not accessible.

Changes proposed in this pull request:

That is triggered by setting the key "fallback-url" for the element like this:

"tool" : {
    "url" : "https://not-accessible-url/tool.phar",
    "fallback-url" : "https://fallback-url/tool.phar"
}

In case the provided url is not accessible, the code will check for the fallback-url key and if set, the fallback url will be checked for accessibility and used to download the asset. Otherwise it displays the error message At least one given URL are not accessible!.

Fixes: #

[tommy-muehle]

Thanks for the contribution!