internal/rosa: improve nil-exec error reporting

This panics with a more descriptive error instead of relying on the SIGSEGV handler.

Signed-off-by: Ophestra <cat@gensokyo.uk>

internal/rosa/state.go

@@ -994,6 +994,13 @@ type evalContext struct {
 	t Toolchain
 }
 
+// HelperError describes a package with nil helper.
+type HelperError string
+
+func (e HelperError) Error() string {
+	return "package " + strconv.Quote(string(e)) + " has no exec helper"
+}
+
 // pf implements [azalea.PF].
 func (ctx *evalContext) pf(
 	name azalea.Ident,
@@ -1172,6 +1179,10 @@ func (ctx *evalContext) pf(
 		}
 	}
 
+	if helper == nil {
+		panic(HelperError(name))
+	}
+
 	v = cachedArtifact{&meta, ctx.t.New(
 		meta.Name,
 		meta.Version,