Improve pkg/registry/service code coverage.

As per #917, I'm trying to learn kubernetes code base by improving pkg/registry/service code coverage.

This commit increase coverage of the package to around 81%.  There are quite a few small branch tests that accumulate to 10%+, I don't if it worth the effort to test.

One more thing about the function TestServiceRegistryList, do I need to test various cases for different labels? since now it only covers labels.Everything()...  It might be necessary unless it is covered by integration test.

Any comments are welcome :)
@lavalamp The reason for lowercase returned error is that we usually print our error message as log.Print("Reading %s: %v", filename, err).
I think the general rule for printing out message should be starting with capital letter. 

Reference: https://code.google.com/p/go-wiki/wiki/CodeReviewComments#Error_Strings