iv: tolerate/display partially-written exr files (originally #1191).

[lgritz]

The idea is to make sure that iv can accept a partial file, meaning: one that opens, but some scanlines/tiles succeed when read and others fail, it should be tolerant of this, and optionally may want a display mode where it's clear which tiles are missing versus black.

The iv side is probably straightforward. The question is how many things you'll bump into upstream (in ImageBuf? ImageCache? ImageInput? OpenEXR's ImageInput implementation?) that might stand in your way and need some adjustment to tolerate the missing parts of the file. Those will need to get fixed.