Improve the error message for bad YAML

[smlx]

Attempt to generate some more easily human-parseable error messages for invalid .lagoon.yml.

This change tries to address #8 by making it easier to figure out exactly what the problem is.

For the .lagoon.yml in #8 it produces an error like this:

    lint_test.go:71: couldn't unmarshal testdata/invalid.4.lagoon.yml: error unmarshaling JSON: while decoding JSON: json: cannot unmarshal array into Go struct field Lagoon.environments of type lagoonyml.Environment.
        There appears to be invalid YAML in the `environments` field:
        
        command: echo "broken definition"
        name: a cronjob defined as environment
        schedule: '* * * * *'
        service: cli

Because it uses the built-in JSON decoding errors this should automatically produce comprehensible errors when extending the .lagoon.yml validation.

[smlx]

Annoyingly depending where the parsing chokes we can hit golang/go#44811 and generate garbage output due to the Offset being incorrect.

Not sure if that matters?