message: make Excepton not use an std::string internally
diff: #define VELOCYPACK_EXCEPTION_H 1
  #include <exception>
  -#include <string>
  #include <iosfwd>
  #include "velocypack/velocypack-common.h"
  @@ -76,17 +75,16 @@ struct Exception : std::exception {
  private:
  ExceptionType _type;
  - std::string _msg;
  + char const* _msg;
  public:
  - Exception(ExceptionType type, std::string const& msg)
  - : _type(type), _msg(msg) {}
  + Exception(ExceptionType type, char const* msg) noexcept : _type(type), _msg(msg) {}
  - Exception(ExceptionType type, char const* msg) : _type(type), _msg(msg) {}
  + explicit Exception(ExceptionType type) noexcept : Exception(type, message(type)) {}
  - explicit Exception(ExceptionType type) : Exception(type, message(type)) {}
  + Exception(Exception const& other) noexcept : _type(other._type), _msg(other._msg) {}
  - char const* what() const noexcept { return _msg.c_str(); }
  + char const* what() const noexcept { return _msg; }
  ExceptionType errorCode() const noexcept { return _type; }
  