Improve error message readability

Test: n/a
Change-Id: Iceb0b68f4b83e2b43c0947885c514252a6de30c1
diff --git a/android/config_bp2build.go b/android/config_bp2build.go
index 2beeb51..b632e33 100644
--- a/android/config_bp2build.go
+++ b/android/config_bp2build.go
@@ -201,7 +201,7 @@
 			panic(fmt.Errorf("error expanding config variable %s: %s", k, err))
 		}
 		if len(expandedVar) > 1 {
-			panic(fmt.Errorf("%s expands to more than one string value: %s", variableValue, expandedVar))
+			panic(fmt.Errorf("%q expands to more than one string value: %q", variableValue, expandedVar))
 		}
 		ret = append(ret, bazelConstant{
 			variableName:       k,