ImportField could use better feedback

[NightJar]

This issue isn't completely fleshed out as yet, but during a recent demonstration it was noted that the feedback one gets on a failed upload (or an upload in general) wasn't all that ideal in that we could be waiting for some time with no feedback if an upload was done, slowly ongoing, or failed.

We need to investigate to see if there's something this module does which causes this (and perhaps could be updated to give better feedback) or if this is a core issue in a similar vein to silverstripe/silverstripe-asset-admin#708