RFR: JDK-8247352: improve error messages for sealed classes and records

[Vicente Romero]

Hi,

Please review the fix for [1] at [2]. The patch is basically changing 
some error messages for records and sealed classes trying to make them 
easier to digest by the user. The bug entry at [1] list some code 
snippets that motivated this fix,

Thanks,
Vicente

[1] https://bugs.openjdk.java.net/browse/JDK-8247352
[2] http://cr.openjdk.java.net/~vromero/8247352/webrev.00/