Improving the error message in the case where the configuration option name wasn't supplied in the web interface for managing generic configuration.

git-svn-id: http://mantisbt.svn.sourceforge.net/svnroot/mantisbt/trunk@4250 <a class="text" href="/?p=mantisbt.git;a=object;h=f5dc347c">f5dc347c</a>-c33d-0410-90a0-b07cc1902cb9