Improve clUpdateMutableCommandsKHR error wording

[EwanC]

As raised in a PR review the error wording around clUpdateMutableCommandsKHR() could be improved.

There are many errors that can be returned here based on errors returned by core OpenCL entry-points, as well as invalid usage of the entry-point itself. This makes the current language quite dense, we should improve the readability.

One idea is to categories errors in the description of commands such that we could refer to the categories later on.