Further cleanup of exceptions in Git API

- Translate internal exceptions to corresponding API exception
- Do not catch GitAPI exceptions internally to an internal
exception. Just pass them to caller
- Mention thrown exceptions in javadoc

Change-Id: I9044cf86d2b0bcc8b63b7cc016e1bf0055a62053
Signed-off-by: Matthias Sohn <matthias.sohn@sap.com>