Make exception messages more consistent.

Make exception messages more consistent.

In particular, change the following two things: (i) Say 'Additional information'
instead of 'Additional Information' because the rest of the text is also
capitalized in the common way. (ii) Indent the actual (additional) error text
by four characters, just like the other additional pieces of information.

The result is that errors now look like this:

--------------------------------------------------------
An error occurred in line <710> of file </home/fac/f/bangerth/p/deal.II/1/install/examples/step-6/step-6.cc> in function
    int main()
The violated condition was:
    false
Additional information:
    abcdef

Stacktrace:
-----------
#0  ./step-6: main
--------------------------------------------------------