apex dependency error: note 'apex_available'

This error tracks `apex_available` closure, but the message doesn't
indicate this variable specifically. Calling it out to try to make
the problem more searchable/discoverable/intuitable.

Bug: N/A
Test: apex_test.go
Change-Id: Ib254ace1dbd4e77d073ed4f98ee181dd86adfcfc
diff --git a/apex/apex_test.go b/apex/apex_test.go
index c52fd04..38dd463 100644
--- a/apex/apex_test.go
+++ b/apex/apex_test.go
@@ -4558,7 +4558,7 @@
 
 func TestApexPropertiesShouldBeDefaultable(t *testing.T) {
 	// libfoo's apex_available comes from cc_defaults
-	testApexError(t, `requires "libfoo" that is not available for the APEX`, `
+	testApexError(t, `requires "libfoo" that doesn't list the APEX under 'apex_available'.`, `
 	apex {
 		name: "myapex",
 		key: "myapex.key",
@@ -4592,7 +4592,7 @@
 
 func TestApexAvailable_DirectDep(t *testing.T) {
 	// libfoo is not available to myapex, but only to otherapex
-	testApexError(t, "requires \"libfoo\" that is not available for the APEX", `
+	testApexError(t, "requires \"libfoo\" that doesn't list the APEX under 'apex_available'.", `
 	apex {
 		name: "myapex",
 		key: "myapex.key",
@@ -4627,7 +4627,7 @@
 
 func TestApexAvailable_IndirectDep(t *testing.T) {
 	// libbbaz is an indirect dep
-	testApexError(t, `requires "libbaz" that is not available for the APEX. Dependency path:
+	testApexError(t, `requires "libbaz" that doesn't list the APEX under 'apex_available'. Dependency path:
 .*via tag apex\.dependencyTag.*name:sharedLib.*
 .*-> libfoo.*link:shared.*
 .*via tag cc\.libraryDependencyTag.*Kind:sharedLibraryDependency.*