Move traits/error_reporting to its own module

[cengiz-io]

Hello @nikomatsakis
As you suggested, we should move traits/error_reporting.rs to a separate file.

This issue is created to serve as a reminder.

[Mark-Simulacrum]

Note: This issue is about moving specific functions out of the error reporting file, see this comment for more details.

[Mark-Simulacrum]

This was done in 7b9519a.