better error reporting details to stderr

[JeanMertz]

Minor improvement when debugging templating errors. The output now prints the final compiled charts.yml, including line numbers, and highlights the problematic area:

Note: in the above case, the problem is actually two lines above the red one, but we can only relay what the YAML parser tells us, and even though the problem originates two lines higher, the actual parsing error comes from the red line, nothing we can do there, except use some common sense to see what's really the cause (also, that's where the extra context helps out)

[niels-s]

afaik