Change "Load successful" pop-up to reflect partial loads

[philrz]

Repro is with Zui commit a372438.

The attached video is from the verification of #2554. Right now when a load partially succeeds, the user is simultaneously shown messages that say "Ingest failed with warnings" and "Load successful". Since "load" and "ingest" are roughly synonymous, this seems confusing. It would be helpful if the "Load successful" could be adjusted in this case to reflect that the load only partially succeeded and that the user is seeing a subset of the data they tried to load.

Verify.mp4

[philrz]

As shown in the attached video, this issue was effectively fixed by the changes in #2834 since "partial loads" are no longer possible in Zui. When a file is staged in Preview & Load, any attempt to load as a format that zq can't parse will prevent the Load button from activating. And if Preview is bypassed by holding down Shift when dragging files into the app, the whole load operation will fail and an empty pool will remain.

Verify.mp4

Zed issue #4546 tracks the wider topic of when support might be added for "partial loads", and surely we'll want good error reporting to be a part of that.