Pull mirroring doesn't clear error message on successful mirroring

Summary

Pull mirroring re-uses the project import data structures for much of its work.

If an error occurs in a pull attempt, this is stored in projects.import_error and displayed. However, this field is not cleared (although it is not displayed) if a subsequent attempt succeeds

Steps to reproduce

Fail a mirror (bad credentials, perhaps) then allow one to complete successfully

What is the current bug behavior?

import_error: "blah blah blah"

What is the expected correct behavior?

import_error: nil

Possible fixes

(If you can, link to the line of code that might be responsible for the problem)