checkfluidsystem: throw exception instead of doing an assert. Considering the...

checkfluidsystem: throw exception instead of doing an assert. Considering the commonly used interface for a component, the exception pretty much has to be thrown for any incompressible fluid phase. One should adapt the check from that perspective.

git-svn-id: svn://svn.iws.uni-stuttgart.de/DUMUX/dumux/trunk@11370 2fb0f335-1f38-0410-981e-8018bf24f1b0