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
I'm not sure if it's very clear for the user of toAbsolutePath that a non-existing path will result in an empty string. I'd have expected an error or a non-existing but absolute path.

At line 30, if the path doesn't exist, jsdoc will be called with one less argument. I'm not sure how the jsdoc parser will react to that.