Fix `go vet` issues

Test: go vet ./...
Change-Id: Ifb936ccc5e2b5a2c3fcbbbcb54f680e2973ea1b3
diff --git a/ui/build/environment.go b/ui/build/environment.go
index 8589937..cbeeb4b 100644
--- a/ui/build/environment.go
+++ b/ui/build/environment.go
@@ -134,7 +134,7 @@
 		if cmd[0] == "unset" {
 			str, ok := singleUnquote(cmd[1])
 			if !ok {
-				fmt.Errorf("Failed to unquote kati line: %q", text)
+				return fmt.Errorf("Failed to unquote kati line: %q", text)
 			}
 			e.Unset(str)
 		} else if cmd[0] == "export" {