jython: fail fast if OMERO_HOME not set

Otherwise the error message ("No module named imagej") is mysterious.

Unfortunately in practice people may need to hack the jython launch
script to hardcode OMERO_HOME, since simply adding it to .bashrc or
.profile may not be sufficient, as those config files may not be
sourced when jython is invoked from OMERO.
README.md: tweak script params and launch commands

This makes them slightly shorter. Still scary though since scripts must
be invoked by ID...