This augments the previous change for issue #3346 by adding an error
message when an invalid integer is seen. This change is likely to be
controversial so I'm not going to squash it into the previous change.
Previously 'set -ql' would only look for variables in the
immediate local scope. This was not very useful. It's also
arguably surprising, since a 'set -l' in a function, followed
by a 'set -ql' in a child block, would fail. There was also no
way to check for a function-scoped variable, since omitting the
scope would also pull in global variables.
We could revisit this and introduce an explicit function scope.
Fixes#2502
