Missing virtual dtor in failure_formatter

[OpatrilPeter]

Hello,
compiling with clang++ 6.0 triggers a warning about not having virtual destructor in failure_formatter while also having virtual functions.
Despite the fact that it currently shouldn't result in any bugs, it could lead to obscure problems later once any type inheriting from failure_formatter would contain any data member requiring destructor.
Even aside from that, this is also a problem for all people treating warnings as errors.

Thus, my recommendation is simple addition of following line
virtual void ~failure_formatter() {}

[sbeyer]

Thanks for reporting! I will fix this soon.