CDS log should print specific reasons for loading failures

When a class specified in the classlist file cannot be loaded because the classfile is of an old, unsupported version, "java -Xshare:dump" prints the following to stderr:

Java HotSpot(TM) 64-Bit Server VM warning: Pre JDK 6 class not supported by CDS: 49.0 FooBarClass

However, it also prints out the following to UL, which by default is in stdout:

[0.602s][warning][cds] Preload Warning: Cannot find FooBarClass

This is confusing, as we did find FooBarClass.

==========
Suggested fix: remove the first warning message. Change the second warning message to

[0.602s][warning][cds] Cannot load FooBarClass - Pre JDK 6 class not supported