block in catalog if someone tries to add something early

fixes the intermittent error we've been getting, such as:

https://buildhive.cloudbees.com/job/brooklyncentral/job/brooklyn/2413/io.brooklyn$brooklyn-core/testReport/brooklyn.entity.rebind/RebindCatalogEntityTest/testRestoresAppFromCatalogClassloader/

does so by blocking if the catalog is not yet fully loaded when someone tries to add something manually to it.  there are better fixes as noted in the code (manual additions could be done in bg) but this is a quicker pragmatic workaround.
I'm indifferent here.
I personally like the curly braces (particularly given two lines lower it is using them).
But there are quite a few places where we don't have braces around one-line conditionals.
If ever it is two lines (because the first is a comment!) then I think we *must* have curly braces.

However, I know @grkvlt you suggested we follow the google style guide (http://google-styleguide.googlecode.com/svn/trunk/javaguide.html#s4.1-braces) - I don't think any consensus was reached on brooklyn-dev about that though.