R1:a1f5d37357d6

Improve the behavior of PhabricatorFileEditField for Macros

Summary:
See D17848. This improves things a little bit in two cases:

Case 1:

- Create a macro.
- Pick a valid file.
- Pick an invalid name.
- Submit form.
- Before patch: your file is lost and you have to pick it again.
- After patch: your file is "held" in the form, you just can't see it in the UI. If you submit again, it keeps the same file. If you pick a new file, it uses that one instead.

Case 2:

- Apply D17848.
- Delete…