Reorganize script logic for easier debugging

[alexander-bauer]

Hey @mirioeggmann, thank you for publishing this project -- I use it in my home environment, and it's saved me a lot of trouble.

I updated today and didn't RTFM, and so didn't notice that I needed to adjust some of the variables I supply. I took a crack at adjusting the script to be a bit more verbose, and to fail at earlier stages if the variables it requires aren't supplied.

I realize this is a substantial reorganization of the whole core logic, but hopefully you find it to your liking.

Thanks again! Sasha

[alexander-bauer]

Some explanatory notes I neglected to include above:

• I changed existing echos to use 1>&2 to redirect output to stderr as opposed to stdout; this doesn't much matter here, but keeps output from the script of operator interest consistently on stderr, and programmatic output on stdout.
• I changed [[ -e "${MYVAR+x}" ]] style tests to [[ -v MYVAR ]], to make them somewhat more succinct.
• I rearranged the auth header logic to test first for API_TOKEN, and then for both AUTH_EMAIL and AUTH_KEY, and fail loudly if both cases fail. I also extracted this to its own section which sets an AUTH_HEADERS array, which is assembled into the final command at the end.
• I made the IP and PAYLOAD determination/assembly more verbose, echoing each.
• I de-duplicated the invocation of the final command by placing its arguments into an array, and then executing it once. Hopefully this will reduce maintenance cost if there's ever another API version change.

[mirioeggmann]

Hey @alexander-bauer

Glad to hear that it is being used & thanks a lot for the contribution!🚀 I think these are definitely some reasonable changes. I will merge the PR and create a new release in the coming weeks.

Cheers Mirio