Generic error resolution schema

[Tomboyo]

There are a variety of circumstances we may or may not foresee in which we can't generate code, such as when the specification causes name conflicts. As a user, I do not want to modify the OAS locally because I'd be responsible for complex three-way merges with the source of truth, but if I could write "rules" in a file that override the OAS in order to address specification flaws, that would let me work around issues without compromising the stability/repeatability of my builds.

Consider how such a framework could work, and what categories of problems it could solve. Can Lily emit exceptions that describe the issue and instruct the User how to fix them?

This might replace #86, #90, and #91.