Adds missing exit value checks in install script

[gurka]

So this is basically just some additional return / exit value checks in the install scripts to make it more robust. I found most of these problems when trying to install IncludeOS under Debian stable. Most of the build steps failed but the script kept going, and finally outputted "Done! Test your installation with ./test.sh", which was kind of .. not so good. This commit partially solves #620 .

I removed the "sudo" part from the README since the script will use sudo when needed. Using sudo when executing install.sh will also build and install everything as root:root and that is probably not want most users want.

I'm not sure if the check for "sudo" is necessary since all supported systems have sudo installer per default (?). Debian does however not, and that's why I added it.

[jenkins-includeos]

Can one of the admins verify this patch with one of the following commands:

• "test please" for a one time test run
• "ok to test" to accept latest and future commits on this pull request for testing
• "add to whitelist" to add the author to the whitelist

[AndreasAakesson]

Yeah, sudo should not be used with install.sh - because of the reason you state; everything gets build and installed as root. Thanks for the PR :)

ok to test

[AndreasAakesson]

retest please

[mnordsletten]

This looks great. Having it exit at the point of failure will make it a lot easier to identify the error.