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 (or only valid to a subset of OS), and due to non existent integration tests this was probably never detected.

Complete debug() dump of the upload post data of this key (OS Win8, PHP5.4):
```php
array(
	'name' => 'codes.csv',
	'type' => 'application/octet-stream',
	'tmp_name' => '...\tmp\php6F13.tmp',
	'error' => '0',
	'size' => '20000'
)
```

var_dump()
```
array (size=5)
  'name' => string 'codes.csv' (length=9)
  'type' => string 'application/octet-stream' (length=24)
  'tmp_name' => string '...\tmp\phpB228.tmp' (length=27)
  'error' => string '0' (length=1)
  'size' => string '20000' (length=5)
```