Factor "X not available when isolation disabled" into common function

[RalfJung]

@christianpoveda added that same check half a dozen times or so. I think it would make sense to factor that into a small helper function that's called everywhere to avoid duplication.

The error message should also mention -Zmiri-disable-isolation so that people know how to make the error go away.

[pvdrz]

Should this go in src/helpers.rs as an standalone function? this doesn't need anything from ecx to be inside the extension trait

[RalfJung]

Well it needs to access the "are we in isolation" flag, right?

[pvdrz]

Oh yeah I was thinking in just factoring the error, not the isolation check. Time to work

[RalfJung]

Ah I see. I meant all the common code, including the check.