Convert a soft error to have a specified (mv erp val state)
See er for relevant background.
(convert-soft-error erp val form)
where form evaluates to an error-triple. The macro call above
is equivalent to form, except that if form evaluates to (mv erp0
val0 state) where the value of erp0 is not nil and val0 is
arbitrary, then erp and val are evaluated and their respective
values e and v are returned in the resulting error-triple (mv erp
val form). Note that it is legal for erp to have value nil, in
which case that error-triple will not designate an error after all.