Only warn about duplicate script engines

Once we have a Python script engine based on real Python (started in
https://github.com/scijava/scripting-cpyton), we will want to avoid
erroring out when there are *two* implementations for the same name,
and instead let the implementation with the higher priority win.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Merge pull request #65 from scijava/duplicate-script-engines

Only warn about duplicate script engines