Unsuccessful glLinkProgram call behaviour

The GLES spec says:

```
If that program object that is in use is re-linked unsuccessfully, the link status
will be set to FALSE, but existing executable and associated state will remain part
of the current rendering state until a subsequent call to UseProgram removes it
from use. After such a program is removed from use, it can not be made part of the
current rendering state until it is successfully re-linked.
```

ANGLE has ran into security problems with unsuccessful links, where our state objects become partially rewritten and use outdated link information. We should fireproof ANGLE, but seeing as there's a risk we could leave multiple hard-to-discover and potentially dangerous security holes I suggest we alter this wording to make WebGL reset the current installed executable when a program that is in-use is re-linked unsuccessfully.

Start by reading the GLES specification passage quoted in the issue and the surrounding WebGL specification text. Determine how unsuccessful relinking of an in-use program should be described, then update the relevant specification wording so the intended current executable behavior is unambiguous.

Written by the indexing model from the issue text.