Fix uploadError validation rule

The error inside the upload array contains `(string)0` which would always fail due to the strict comparison against an integer 0.
This resolves it.

The initial test cases were an incorrect assumption, and due to non existent integration tests this was probably never detected.

Complete debug() dump of the upload post data of this key:
```php
array(
	'name' => 'codes.csv',
	'type' => 'application/octet-stream',
	'tmp_name' => '...\tmp\php6F13.tmp',
	'error' => '0',
	'size' => '20000'
)
```