Distinguish between Jsonnet parse and runtime errors

[lawrencejones]

Just had a funny realisation that when we load files and try Jsonnet first, if Jsonnet fails because of a runtime error (as opposed to, for example, a parser error) then we'll fallback to YAML.

Sadly, YAML will accept almost anything, including valid Jsonnet. So we end up awkwardly parsing Jsonnet as YAML which is unlikely to be what people want at all 😂

We should ensure that if the Jsonnet error is a runtime error we don't try parsing it as YAML.

Example is:

{
  key: error 'oh no',
}

Which YAML will parse happily as:

{
  "key": "error 'oh no',"
}