Allow users to respond to unsatisfiable and mutually exclusive constraints

[jspahrsummers]

Since Arbiter knows specifically which requirements were unsatisfiable or in conflict, it should ask the user what to do, instead of just blindly erroring out.

This could then be used to "resume" the dependency resolution process, by assuming that the user-given solution is valid.

We should only do this with the actual fatal error, though—this shouldn't happen if a solution may still exist.