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.go b/apex/apex.go
index a9a58a6..25cd09d 100644
--- a/apex/apex.go
+++ b/apex/apex.go
@@ -2075,7 +2075,7 @@
 		if to.AvailableFor(apexName) || baselineApexAvailable(apexName, toName) {
 			return true
 		}
-		ctx.ModuleErrorf("%q requires %q that is not available for the APEX. Dependency path:%s", fromName, toName, ctx.GetPathString(true))
+		ctx.ModuleErrorf("%q requires %q that doesn't list the APEX under 'apex_available'. Dependency path:%s", fromName, toName, ctx.GetPathString(true))
 		// Visit this module's dependencies to check and report any issues with their availability.
 		return true
 	})