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Add FORWARDTEST.BADINPUT.RECOVERABLE

Change positive_numeric_arg to positive_flonum_arg, following nonnegative_flonum_arg().
Instead of returning a NODE*, it returns a FLONUM. This is simpler for the caller.

Instead of being parametrized by the list, it takes the argument to convert.
Therefore, it no longer has the side effect of modifying the argument list.

This is part of a larger effort to reduce the number of side effects in the code base.

Add more test coverage for when ZOOM is given bad input.

Rename nonnegative_flonum_arg to nonnegative_flonum_arg.
Also, change it to take the argument, instead of modifying the argument list.
This follows the example of flonum_arg and makes it easier to call,
as the caller no longer has to extract the FLONUM from a numeric NODE.

This is part of a larger effort to reduce the number of side effects within the code base.

Fix a test bug in WAITTEST.BADINPUT.RECOVERABLE.
It incorrectly and misleadingly reported that WAIT output a value, when the value came directly from :erract because WAIT (correctly) didn't output one.

Fix an incorrect claim in the comment for flonum_arg.

Add more regression tests for WAIT, particularly for bad input and recovery of bad input.

Fix a typo in a test failure message.

Add comments to explain a mysterious block of code in evaluator() as well as a test that exercises it.

Add tests for SETPOS.