cmd/gomobile: fix error message to reflect current build target

Change-Id: I94b7d1c7029102c252a77f75929066281040b9f2
Reviewed-on: https://go-review.googlesource.com/c/mobile/+/564856
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Emmanuel Odeke <emmanuel@orijtech.com>
Reviewed-by: Hyang-Ah Hana Kim <hyangah@gmail.com>
Auto-Submit: Hyang-Ah Hana Kim <hyangah@gmail.com>
diff --git a/cmd/gomobile/build.go b/cmd/gomobile/build.go
index c948343..f8895eb 100644
--- a/cmd/gomobile/build.go
+++ b/cmd/gomobile/build.go
@@ -160,7 +160,7 @@
 			return pkg, nil
 		}
 		if buildBundleID == "" {
-			return nil, fmt.Errorf("-target=ios requires -bundleid set")
+			return nil, fmt.Errorf("-target=%s requires -bundleid set", buildTarget)
 		}
 		nmpkgs, err = goAppleBuild(pkg, buildBundleID, targets)
 		if err != nil {