Review Request: JDK-8174694: improve error message shown when main class can't be loaded

[Vicente Romero]

Hi,

Please review the fix for bug [1]. The fix is improving an already 
existing error message. The public patch can be reached at [2]

Thanks,
Vicente

[1] https://bugs.openjdk.java.net/browse/JDK-8174694
[2] http://cr.openjdk.java.net/~vromero/8174694/webrev.00/jdk.patch