AssetAdmin does not report any _unexpected_ errors, and just stops.

[NightJar]

SilverStripe\Assets\Upload_Validator::validate runs a series of checks and returns false if one fails (true otherwise).
This appears to work reasonably well in common use cases (such as a file being over the legal limit as such), however does not if for example we have a custom rule. The file is sent off, and a perfectly valid response is sent back, however there is precisely zero feedback from the AssetAdmin, and it refuses to do any more business with invalid (or sometimes valid) uploads.

@unclecheese advises me that the current checks in Upload_Validator are recreated and performed on the client side. Simply prepending the validate method with

$this->errors[] = 'Test bad response';
return false;

is enough to hack in an always repeatable occurrence of the error.

tl;dr AsssetAdmin ignores all server side validation errors.

[unclecheese]

Resolved here: #709

[tractorcow]

I'm reviewing the fix.

[tractorcow]

The fix is good and can be merged on green.