RFR[XS] 8191374 Improve error message when CDS is not supported on exploded build

[Ioi Lam]

Hi, please review this very simple fix for JDK 11:

https://bugs.openjdk.java.net/browse/JDK-8191374
http://cr.openjdk.java.net/~iklam/jdk11/8191374-exploded-build-dump-error-msg.v01/

Summary - the error message is changed to be more obvious what's going on.


Testing: since we don't have automated tests for exploded builds, I just 
did some
manually testing on Linux using the "exploded build", ./jdk/bin/java, 
which is an
intermediate file created by the JDK build system.


Before:

     $ ./jdk/bin/java -Xshare:dump
     Error: non-empty directory '/jdk/bld/opencds/jdk/modules/java.base'
     Hint: enable -Xlog:class+path=info to diagnose the failure
     Error occurred during initialization of VM
     CDS allows only empty directories in archived classpaths


After:

     $ ./jdk/bin/java -Xshare:dump
     Error occurred during initialization of VM
     CDS is not supported in exploded JDK build


Thanks
- Ioi