Guard against serializing non existent folders that crash the build.

[Jeff Schwartz]

When cloning a fusion.ssg project, there is a chance that numerous src folder may not exist if the repo from which the project was cloned did not have them, such as the data. css or media folders for example. When this occurs and the user builds the project, the build will crash during serialization because it is attempting to copy these folders to the build folder.

To address this issue, always check if the folder exists prior to serializing it to the build folder.