Linux script updates for additional error checking

[corttompkins]

Thanks for the script, very helpful to not have to deal with a Windows VM to get my new MiSTer going!

I ran into some issues because I didn't have mkfs.exfat on my Ubuntu 18.04 desktop. This PR adds extra error checking and message formatting. This should address 2 of the 3 things in issue #4 (I didn't encounter problem 3).

Details:

• Add separators to distinguish between script status messages and Linux command output.
• Trap non-zero exit codes and immediately die with an error message.
• Check for additional prerequisite: mkfs.exfat
• Added extra checks to validate temporary mnt and download directories.
• I long ago lost the ability to write scripts without bashisms, so the interpreter is now bash. Hopefully this seems reasonable for an end-user/enthusiast helper script :).

[michaelshmitty]

This is great work @corttompkins , thanks! I'll merge this right away. I haven't found time yet to try out the Linux version of the script. When I do, I'll see what I can backport to the mac version because this is much cleaner. I too am glad not having to spin up a VM or dig up a windows laptop somewhere to get the MiSTer SD card kickstarted! Thanks and have fun with MiSTer!