readme and plugin checks should use the same error code

[kkmuffme]

e.g. plugin_header_nonexistent_requires_wp and nonexistent_tested_upto_header are checking the same thing, while 1 is for the plugin and the other for the readme.
There are other similar ones e.g. for license,...

If the same thing is checked, the same error code should be used.

Given the fact they use different error codes, I assume there's probably some duplicate code that can be deduplicated in the plugin itself too

[ernilambar]

To avoid code duplication, we have used Traits and helpers for but yah, we did not change the error code for historical reason. For plugin header checks, most of the checks error code is prefixed with "plugin_header_".

[ernilambar]

Marking as "May be later".