Improve error on not provided parameters

[matteo-cristino]

Since the improvements in error reporting done in slangroom one thing (along with zenroom error that are next to be done) has been left behind. The error returned by fetch/fethOpne/fetchonnect https://github.com/dyne/slangroom/blob/de0850af5fbd15b5996a55a72aab551ea4fecd33/pkg/core/src/plugin.ts#L535-L542

In case a variable used by a statement, that uses fetch, is not found the error that the user would see is just

Error: the parameter isn't provided: ${parameters_name}

In order to have better error reporting (with lines, colour and so on) also in this case the only thing that I see is to modify all ctx.fetch with

fetch(lhs: string): PluginResult {
    const val = this.get(lhs);
    if (!val) return {ok: false, error: new MissingVariableError(`the parameter isn't provided: ${lhs}`)});
    return {ok: true, value: val};
 }

and modify where it is called with

const fetchRes = ctx.fetch("value_name");
if (!fetchRes.ok) then return ctx.fail(fetchRes.error);

In the same way it will be done for fetchOpen and fetchConnect.

Any better idea? 💡

[puria]

No it's ok to me to propagate the error, since we can't know how the fetch sentences will be implemented in plugins

[matteo-cristino]

We could do somithing like this in slangroom.ts

try {
  // slangrom_execute_statement
} catch(e) {
  if (e instanceof FetchError) {
    // throw nice colored error
  }
  throw e; // left for wrong no catched error in the plugins (should not happen)
}

or is it not worth it in you opinion?

[puria]

IMHO not needed