Improve error output for missing ABI dumps.

Bug: http://b/156513478
Test: tried using this, saw the error message
Change-Id: Ia5b292315a5c5d603ab0ba4dc3303b1b8ff2ef5b
diff --git a/cc/ndk_library.go b/cc/ndk_library.go
index d704e32..2473ba2 100644
--- a/cc/ndk_library.go
+++ b/cc/ndk_library.go
@@ -396,14 +396,14 @@
 }
 
 func (this *stubDecorator) diffAbi(ctx ModuleContext) {
-	missingPrebuiltError := fmt.Sprintf(
-		"Did not find prebuilt ABI dump for %q. Generate with "+
-			"//development/tools/ndk/update_ndk_abi.sh.", this.libraryName(ctx))
-
 	// Catch any ABI changes compared to the checked-in definition of this API
 	// level.
 	abiDiffPath := android.PathForModuleOut(ctx, "abidiff.timestamp")
 	prebuiltAbiDump := this.findPrebuiltAbiDump(ctx, this.apiLevel)
+	missingPrebuiltError := fmt.Sprintf(
+		"Did not find prebuilt ABI dump for %q (%q). Generate with "+
+			"//development/tools/ndk/update_ndk_abi.sh.", this.libraryName(ctx),
+		prebuiltAbiDump.InvalidReason())
 	if !prebuiltAbiDump.Valid() {
 		ctx.Build(pctx, android.BuildParams{
 			Rule:   android.ErrorRule,