box models: improve exception handling

all non-NumericalProblem exceptions are caught at the level of the
newton method and the constructor of the NumericalProblem class can
directly take a string as an argument for improved convenience,

git-svn-id: svn://svn.iws.uni-stuttgart.de/DUMUX/dumux/trunk@5040 2fb0f335-1f38-0410-981e-8018bf24f1b0