Files which could not have coverage measured should be represented in coverage reports

All of the coverage tools we're likely to use will occasionally have parsing errors on some files and hence won't be able to generate coverage reports.

Previously our solution to this was to manually blacklist these files, while we wait for the coverage tool to be fixed.

Now we have a solution that we automatically recover from failures of this nature: if a file fails to compile when using the coverage tool, we automatically try to compile again without the coverage tool, and print out a warning. (see tests/generic/covtool_wrapper.pl in qa-dungeon)

This warning to the compile log is currently the only indication that data will not be gathered for this file. This is easily missed. The coverage reports themselves should state in a highly visible way, Warning: these sources could not be tested: ... , with a log of the error to help the developer refactor the code to make it work (if the choice is made to do so).