added error handling for invalid plugin versions

Added code to catch exception when a plugin's update properties file
contains an invalid version number.  Checking for updates will now
complete successfully (before, it hung).  Protege logs error in the
"Protege-Guard", so user can click the red triangle to see exception.
Exception is also printed to console window, as well as a more human
readable error message about the invalid version number.  Fixes #18.