fud2 silently fails with errors

[rachitnigam]

When adding a new stage, fud2 will silently fail when there is a problem. For example, I am adding a new Dahlia stage and when the key is absent, instead of giving an error, fud2 generates an incorrect script:

import "calyx" as c;

export const dahlia = state("dahlia", ["fuse"]);

defop dahlia_to_calyx(prog: dahlia) >> out: c::calyx_state {
    let dahlia = config_or("dahlia.exec", "dahlia");
    let flags = config_or("dahlia.flags", "");

    shell(`${dahlia} ${flags} ${prog} > ${out}`);
}

With the command:

fud2 tests/frontend/dahlia/binop_tree.fuse --to calyx -m emit

Will generate:

build-tool = /Users/rachitnigam/.cargo/bin/fud2
rule get-rsrc
  command = $build-tool get-rsrc $out

# dahlia_setup (plugin)
Failed to load plugin: dahlia-to-calyx.rhai
  Emitter error: missing required config key: dahlia

# build targets
build _to_stdout_calyx.futil: dahlia-to-calyx tests/frontend/dahlia/binop_tree.fuse

default _to_stdout_calyx.futil

This should be considered a blocking usability issue for the fud2 transition.