GetField and SetField do not have the same behavior when the accessor is undefined

[progval]

When used on a field with undefined as #getter, an explicit error is displayed:

$ echo "{[] 'foo': {#getter undefined, #setter undefined}}['foo']" | ../obj/s5.d.byte -load init.heap /dev/stdin -continue-s5-eval

Uncaught error: "[interp] (/dev/stdin:1:0-57) Applied non-function, was actually undefined"

Fatal error: exception Failure("Runtime error")

But, when setting the field instead of getting it, does not show an error at all:

$ echo "{[] 'foo': {#getter undefined, #setter undefined}}['foo' = 'bar']" | ../obj/s5.d.byte -load init.heap /dev/stdin -continue-s5-eval
Fatal error: exception Failure("Runtime error")

When reading the source code (at https://github.com/brownplt/LambdaS5/blob/master/src/ljs/ljs_eval.ml#L529), I noticed that it should display an error message, but the eprintf calls seem not to do anything (replacing them with print_string shows Uncaught exception: "unwritable-field"), but I have no idea why…