Improve the error model

[jacquesg]

It's not always possible (or preferred) to deduce exactly what went wrong with a plain old string error. libgit2 can provide the error class and error code in addition to an error message.

• Implement a first class error object that captures the error code, class and message.
• Overload the "" operator to print the error message so existing code just works
• Expose the error code and error class constants to the user

[coveralls]

Coverage decreased (-0.4%) when pulling 3e4f9fe on jacquesg/error-model into b1cbbf8 on master.