The error handling in the ssh certificate callback is straightforward.There's no error messages from an external library that need to besaved, we populate the error message ourselves. Since there's nothingcustom here, it does not need to use the error saving mechanism.

Callers want to be able to simply call `git_error_last()->message`,not have to worry about whether `git_error_last()` returns NULL or not.

Most callers only need to _get_ error messages. Only callers implementedmore complicated functions (like a custom ODB for example) need to setthem.(Callback users should likely ferry their own error information in theircallback payload.)

Instead of having a separate type for saving state, we can re-use the`git_error` structure. In practice, saving the error code along with the`git_error` information has not proven necessary or useful, so it can beremoved to simplify down to re-using a single structure.

Now that thread-local error data is handled in error, move the threadlocal data out of the `threadstate` object, since it has now becomeuseless indirection.

We now have no "big single object" that contains thread state.