NessusXMLRPC: added __str__ to exceptions
nessus: xsltproc fixes

Apparently xsltproc's command line arguments changed since the original
code been written, and as such, it no longer works with current xsltproc
versions.

Due to the lack of error handling regarding xsltproc, this error is hidden
from the user. Error handling and reporting now added.
nessus: try to create a compressed zip file

Python's zipfile module creates uncompressed zip files by default, due
to a a bit legacy configuration setting, because Zlib support is optional.

Since it's not a problem in most modern Python builds, we try to create
the report zip file, with compression enabled, and if it fails, we roll
back to use the old no-compression setting.