Don't panic for unhandled product vars

Instead, we return an error. This allows us to access some product
variable information earlier when it will not be used as an attribute
without panicing

Test: m nothing
Change-Id: Id094b2b9e1364a8d174d99b3824fa149fb235b3e

apex/apex.go

@@ -3298,7 +3298,10 @@ func convertWithBp2build(a *apexBundle, ctx android.TopDownMutatorContext) (baze
 		cannedFsConfigAttribute.SetValue(android.BazelLabelForModuleSrcSingle(ctx, *a.properties.Canned_fs_config))
 	}
 
-	productVariableProps := android.ProductVariableProperties(ctx, a)
+	productVariableProps, errs := android.ProductVariableProperties(ctx, a)
+	for _, err := range errs {
+		ctx.ModuleErrorf("ProductVariableProperties error: %s", err)
+	}
 	// TODO(b/219503907) this would need to be set to a.MinSdkVersionValue(ctx) but
 	// given it's coming via config, we probably don't want to put it in here.
 	var minSdkVersion bazel.StringAttribute