Fixing issues go vet reports.

Running go vet uncovered a few style issues.

1) Several unreachable returns (no longer necessary because hugo requires go >= 1.1)
2) Several creations of source.InMemorySource not using the field's name when creating an instance (really only in tests).
3) A few incorrect types in formatting directives for printf
4) Renaming errorf -> errorln to no longer imply it takes a formatting argument (which confused go vet and anyone using the function).