When NetInstall receives YAML data, handle parser errors more gracefully:
show line and column, but because it's network data (not in a local file),
do some work to print out the actual data received.
FIXES#786
Package groups are divided into critical and non-critical
depending on whether we want all Calamares to fail if installing
a package in the group fails, or we are okay with just logging a
warning.
The distinction is configured in the YAML file listing the package
groups. By default, all groups are critical, to keep supporting
the previous behaviour.
In other words, support:
hidden: false
selected: true
groups.
This was supposed to work according to README.md, but not actually
implemented. Now it should be working.
