[RFR] 8205525 : Improve exception messages during manifest parsing of jar archives

[Baesken, Matthias]

Hello, please review this small change  that  improve exception messages during 
manifest parsing of jar archives .

Thanks, Matthias

  Bug :

https://bugs.openjdk.java.net/browse/JDK-8205525

  Webrev :

http://cr.openjdk.java.net/~mbaesken/webrevs/8205525/