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

etc/prebuilt_etc.go

@@ -730,8 +730,11 @@ func (module *PrebuiltEtc) Bp2buildHelper(ctx android.TopDownMutatorContext) (*b
 				src.SetSelectValue(axis, config, label)
 			}
 		}
-
-		for propName, productConfigProps := range android.ProductVariableProperties(ctx, ctx.Module()) {
+		productVarProperties, errs := android.ProductVariableProperties(ctx, ctx.Module())
+		for _, err := range errs {
+			ctx.ModuleErrorf("ProductVariableProperties error: %s", err)
+		}
+		for propName, productConfigProps := range productVarProperties {
 			for configProp, propVal := range productConfigProps {
 				if propName == "Src" {
 					props, ok := propVal.(*string)