Be more verbose in a warning message.

In order to track down what's happening, knowing which file we were trying to write to turned out to be about as useful as knowing the processor number.
Merge pull request #168 from bangerth/more-verbose-error

Be more verbose in a warning message.

In order to track down what's happening, knowing which file we were trying to write to turned out to be about as useful as knowing the processor number.