[test][cleanup] Remove or reduce exception handlers

We usually have a list of exception handlers like this at the end of each main file

catch (Dumux::ParameterException &e)
{
    std::cerr << std::endl << e << " ---> Abort!" << std::endl;
    return 1;
}
catch (Dune::DGFException & e)
{
    std::cerr << "DGF exception thrown (" << e <<
                 "). Most likely, the DGF file name is wrong "
                 "or the DGF file is corrupted, "
                 "e.g. missing hash at end of file or wrong number (dimensions) of entries."
                 << " ---> Abort!" << std::endl;
    return 2;
}
catch (Dune::Exception &e)
{
    std::cerr << "Dune reported error: " << e << " ---> Abort!" << std::endl;
    return 3;
}
catch (...)
{
    std::cerr << "Unknown exception thrown! ---> Abort!" << std::endl;
    return 4;
}

Small cleanup point:

• Catch all exceptions as const&

Actual discussion points:

• The default terminate handler usually provides the exception message as well so manually catching exception is not necessary
• In case of the Dune::DGFException we actually add more information so that is a sensible use of the exception handler. However not every test will have the chance to throw a Dune::DGFException because not every test uses DGF mesh files.
• In the other cases we just print the exception message which is what the default handler does anyway on most system AFAIK
• Catching (...) suppresses the exception message completely. This is a code smell. When such an exception occurs you have to manually remove the line to be able to read the exception message.

Suggestions:

• Try what is printed when we remove exception handlers completely (probably we don't need handlers at all for tests)
• Think about how to reduce handlers to a minimum
• Remove default catch case (...)

Note: These comments are specifically for tests. If you have some Dumux application you might want to add exception handlers to gracefully exit out of an exception situation.