hint for missing installation packets in error messages

[Alexander Kreutz]

A useability problem:

If Zorba is installated via apt-get the user may need to install additional modules like the http-client.
If the user forgets to do so he receives error messages like:

static error [err:XQST0059]: "http://expath.org/ns/http-client": target namespace not found for schema/module http://expath.org/ns/http-client

A user who uses the system for the first time like me thinks that the internal libraries are not found and tries to correct the library path which is the wrong because the solution is to install additional packages.