Fix dangling pointer and message in error.hpp.

[rec]

Two little problems still in this file.

1. what() returns a const char* that comes from a std::string. If the std::string changes, the const char* might become invalid, resulting in undefined behavior.
2. The first and second constructors of exception are inconsistent. In the first one, if there's no message the message from the error code is used. In the second one, there's no message, even though there's an error code provided.

My solution to the first problem was simply to make that one std::string member const. It will break any code that changes the m_msg on an exception - but if they did that, they were flirting with UB anyway - and everything else should work unchanged.