generateDocs.sh: Simplify

* Make more like mediawiki-core's mwjsduckgen.
  - Use bash set -e to have the script abort early when any
    part of it fails (especially the jsduck command).
  - Since it will abort early, we don't need the 'test' for the
    symlink. If jsduck succeeded, it will have re-created the
    docs directory and thus not have the symlink yet.
  - Don't "cd" into anything from the script, simply use absolute
    paths instead. Both jsduck and ln support this.

Change-Id: I03c02d590e0f696c78da87ff638bb407e48a7cd7
Merge "generateDocs.sh: Simplify"