fix(engine): return the cue values merge error if non nil

[GeorgeMac]

I found a rather confusing error while making changes to some values.cue default definitions, which led me to investigate (happy to open an accompanying issue if that is useful):

10:13AM ERR build failed:
explicit error (_|_ literal) in source:
    ./values.cue:2:9

The change proposed returns the cue Values error if not nil, before attempting to write out the generated value.

The problem is in my values.cue file. It was that I had put a concrete list in my default values that wasn't compatible with my changes in the overriden values. For example:

// in default values.cue:
values: {
  list: []
}

// in overridden values.cue:
values: {
  list: [{ name: "some other entry" }]
}

I figured this had something to do with CUE merging my default and supplied values files together.
The reference ./values.cue:2:9 made no sense relative to my values files, so I assumed it was referring to some other intermediate generated file which led me to find the module builder. When I put in a debug line to print out the finalVal I found in had the contents (literall bottom value with a comment):

_|_ // values.list: incompatible list lengths (0 and 1)

Changing the builder to return the value error if not nil results in timoni failing like so:

10:14AM ERR values.list: incompatible list lengths (0 and 1)

[stefanprodan]

@GeorgeMac thank you for this. Just as a remark that the values.cue from the module root should be empty. The best place to provide defaults, is in the #Config definition. An example for this is in the https://github.com/stefanprodan/timoni/tree/main/blueprints/starter.

[stefanprodan]

LGTM

Thanks @GeorgeMac 🏅