RFR(xs): 8154976: UseSharedSpaces error message is incomplete

[Calvin Cheung]

Please review this small fix for completing the error message in case 
there's a mapping failure when using a CDS archive. This error will be 
shown when the -XX:+PrintSharedSpaces option is used.

Before the fix:
UseSharedSpaces: Unable to map %s shared space at required address.

After the fix:
UseSharedSpaces: Unable to map ReadOnly shared space at required address.

JBS: https://bugs.openjdk.java.net/browse/JDK-8154976

webrev: http://cr.openjdk.java.net/~ccheung/8154976/webrev.00/

Testing:
     JPRT
     manual test on windows to ensure the error message is correct.

thanks,
Calvin