RFR: Try to install already installed jmod/modular jar issue

[Chris Hegarty]

Trivial issue that crept in whereby if you try to install an already 
installed module it will actually remove it!

If a module is already installed ( or there is a view or alias with a 
similar name ) the addition to the module directory will fail. The error 
handling code incorrectly assumes that the requested module should be 
removed.

Trivial change to only attempt removal after the addition to the module 
directory has succeeded, i.e. only if there has been a change to the 
actual library or meta data.

http://cr.openjdk.java.net/~chegar/jigsaw/doubleInstall_webrev.00/webrev/

-Chris.