[4.2] Skip Database Validation If Attribute Has Already Failed

Related to [#6048](https://github.com/laravel/framework/issues/6048)

In Postgres the existence validation raises an exception when comparing a string agains an integer column. This commits skips the database validator when the attribute has already failed, avoiding the exception.
Please add an extra new line between the short and long descriptions, and please end the long description in a full stop.