Avoid crash when inserting macro template in tabular inset

Avoid crash when inserting macro template in tabular inset

The issue here is that macro templates are forbidden in InsetTabular
(why? I do not know) and Text::getStatus does not enforce that
properly. Text::insertInset is called and does nothing (because
insertion is forbidden) and yet the cursor is changed to point into
this non existent inset.

Solution:

1/ block insertion of macro templates when not allowed

2/ (additional safety) when insertion of a math macro inset failed, do
   not try to set cursor inside the non-existing inset.

Additionally clarify comments.