Feature request: Exceptions should give more descriptive error messages

[muesli4]

Just giving the class name is not really useful for the user. Otherwise it is a lot of work for a programmer to get all the details out of the exact exception type (or even list all of them in catch clauses).

Proposed solutions:

• If support for C++11 is available: Inherit from std::runtime_error and use delegating constructors to initialize it with the correct error message (e.g. depending on path etc.).
• Otherwise: Add a protected std::string member to class ConfigException and after all members are initialized set it to the error message for the particular class.

I will probably look into this myself, if you accept pull requests.