Do not try to generate documentation when the required apiDocPath option is missing

I don't feel comfortable just dumping the docs in some directory, so srcPath and apiDocPath are both required and jester will just skip generating documentation when thery are missing.

Also improved:

- fixes typo in jester-batch
- check for srcPath existence in rebuildDocumentation
- do not try to use a falsy path var in rebuildDocumentation