cerrno exception class

[Peter]

Standard clib often returns non-zero to indicate an error and sets global variable errno to indicate type of error. In C++ it often preferable to communiate error by exceptions instead and therefore I would like an exception class

class {
public:
my_exception(std::string msg);
}

for which the error message (msg) is appended with the output of strerror(errno).

[Peter]

(In [2209]) added an errno exception class (fixes #616) and implementing exception classes in source file (rather than inlining).