Consistently Throw NotSupportedException on Enumerator Reset

[JonHanna]

Fixes #2879

Have Enumerable.Iterator's implementation of IEnumerator.Reset throw
NotSupportedException instead of NotImplementedException as both more
appropriate in itself, and more consistent with other IEnumerator.Reset
implementations.

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[stephentoub]

cc: @VSadov

[stephentoub]

If this is a change we want to make, it LGTM. @VSadov, do you want to change the exception employed here?

[VSadov]

Compiler-generated Reset throws NotSupportedException, so it makes sense for hand-written substitutes to throw that as well.

I do not think there is a compat issue here since calling Reset on iterators is rare, dependency on a particular exception thrown even more so, in particular since LINQ operators implemented as C# iterators (most of the operators) throw NSE.

[VSadov]

LGTM