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.
Minor edits to some text.
Merge pull request #172 from bangerth/doc-fixes

Minor edits to some text.
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.