Move `Iface.Load` errors into `Iface.Errors` module
This MR moves the error-related functions in GHC.Iface.Load
into a brand new module called GHC.Iface.Errors
. This will avoid boot files and circular dependencies in the context of #18516, in the pretty-printing modules.
The main goal is trying to reduce the footprint of !4798 (closed). Furthermore it feels like standard practice to break up modules based on their purpose, and GHC.Iface.Errors
reflects that.