Changes for when gcovr 4.2 gets released

[tovrstra]

Add the option --exclude-throw-branches to gcovr once gcovr 4.2 is released. C++ coverage analysis is pretty useless without it, because many not so relevant branches are introduced by the compiler to handle exceptions. Most of these branches (or arcs technically) are always missed.

This may not be enough, i.e. other sources of useless branches exist, and branch coverage may have to be disabled entirely to get sensible results for C++ coverage.

[tovrstra]

This got fixed in another way in 1.5.0.