JDK 9 RFR(s): 8169203: (jdeprscan) eliminate duplicate "can't find class" errors

[Stuart Marks]

Hi all,

Please review this small jdeprscan fix for another error message cleanup. This 
time, keep track of which classes aren't found so that a "not found" error is 
emitted only once per class not found. Without this fix, the duplicates can 
overwhelm the rest of the output.

Bug:
	https://bugs.openjdk.java.net/browse/JDK-8169203

Webrev:
	http://cr.openjdk.java.net/~smarks/reviews/8169203/webrev.0/

Thanks,

s'marks