Improve Agent debugging

* Set `:proc_lib` initial call to `Agent`'s init fun
* Translate `Agent`'s :error_logger` messages in `Logger`

This brings `Agent` in line with `Task`, though I am unsure about the logging format.

A side effect of this commit is a different `:sys.get_state` state. Though given `Agent`'s have functions to change the state I do not view this as a problem.