Extend the error message for ExcIndexRange.

Extend the error message for ExcIndexRange.

I must have had half a dozen students this semester who were unable to
decipher what this error message really means:

An error occurred in line <1183> of file
<../include/deal.II/lac/vector.h> in function
    Number &dealii::Vector<double>::operator()(const size_type) [number
= double]
The violated condition was:
    i<vec_size
The name and call sequence of the exception was:
    ExcIndexRangeType<size_type>(i,0,vec_size)
Additional Information:
Index 0 is not in [0,0[

This commit simply provides them with some more information.