Improve the "files not present" error message
It's possible to use -deffnm in restarts even if it wasn't used in
the initial simulation. This can lead to absurd situations such as:
Expected output files not present or named differently:
pullx.xvg
pullf.xvg
where pullx.xvg and pullf.xvg are present and named exactly as listed,
but GROMACS expects them to be named as -deffnm requested.
The improved error message suggest to the user to check for that
possibility.
Refs #942 (partial workaround)
Change-Id: I983a7a2be791a634b877b0cbadb34e56a1ee2f82