C++ Community Plugin silently ignores unknown files in gcovr reports

[wenns]

In an earlier version of the CXX plugin, the coverage analyzer would log a warning message if it encountered a file in the gcovr report that it didn't know about. That has been changed to the debug level, causing silent failures unless running at log level debug. This should be a warning or error, since it is not likely to be what the person running the analysis expects.