Support gzerr and friends

[gerkey]

In the course of migrating a plugin from Gazebo to Ignition (#80), one of the seemingly unnecessary changes was to replace gzerr with ignerr (same for the warn, log, and dbg versions).

Given that the functionality and signature are seemingly unchanged, it would be nice if gzerr and friends just kept working.

If there's a technical challenge to continuing to allow the old names (@chapulina mentioned that there are use cases where gzerr and ignerr are mixed in the same code), then I'm OK with calling this wontfix, given that the cost to the user is a simple search-replace.

[chapulina]

Thanks for bringing this up.

I'm OK with calling this wontfix, given that the cost to the user is a simple search-replace.

We'll go with this for now, and we can revisit if it becomes a common pain point to many migrating users.

---

One thing we should consider more generally would be some migration script that does various search-and-replace tasks, see #98. As a reference, here's a script which, among many things, migrates the console logging macros from ROS 1 to ROS 2.