Better error handling in bootstrap script

[yuvipanda]

Provide nicer error messages, and check for error conditions properly in the bootstrap script.

[yuvipanda]

Things to check:

1. Ubuntu
2. Appropriate version of Ubuntu
3. Git installed
4. Appropriate version of Python
5. Systemd installed and present (accounts for running inside docker container)

[yuvipanda]

From #219 (comment), we should also check for external internet access & fail if we can't reach it. (Thanks @fm75)

[yuvipanda]

We already do the following:

1. Validate it's ubuntu
2. Validate version of ubuntu
3. We install git ourselves, so that's ok

[yuvipanda]

I'm not particularly sure how to check for 'do we have internet access'. I don't want to do something like 'ping google.com' or any particular external website, since that will leak information about the server being installed. I guess we should watch for apt failures and suggest that as a possible solution.

[yuvipanda]

#340 adds validation for python version and systemd presence. It also handles erroring with a more useful error message when running inside docker.

[consideRatio]

I'll close this, we don't check explicitly for internet connection issues, but we covered all the rest listed here:

• Ubuntu
• Appropriate version of Ubuntu
• Git installed
• Appropriate version of Python
• Systemd installed and present (accounts for running inside docker container)