Better download mechanism with aria2c and code beautified

[angoca]

I replaced wget for aria2c, which provides better control for parallel downloads, and to continue when the script got canceled.

Also, I ran shellcheck to improve the robustness of the script.

Finally, I added some bash flags to validate the correct execution.

[markstos]

This patch doesen't appear to do anything to address aria2c being a new dependency, while wget is often available by default.

It could be improved helping to get aria2c installed, by checking if aria2c is installed and throwing a clear error message if not found, and updating documentation to note that a new dependency has been added.

But before doing all that, it would be useful to provide some benchmarks to demonstrate that aria2c provides a meaningful performance improvement to make it worth adding a new dependency.