Make exception messages more consistent.

[bangerth]

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
--------------------------------------------------------

This is a follow-up to #3189.

[tjhei]

Indent the actual (additional) error text
by four characters, just like the other additional pieces of information.

You are only indenting the first line of the additional output though. Wouldn't this look somewhat weird if the additional information wraps over more than one line? (I don't care too much about this so if you feel strongly about it...)

[bangerth]

Yes, only the first line. It's not quite straightforward to indent all lines because we have no control (at this point) over how these lines are formatted.

Let's try this out, and if we find that it looks awkward I'll think of something else.