Fix upper bound on non-strict while

[oskgo]

Ensure that upper bound for variant in non-strict while cannot be written to during the loop.

Fixes #917

[oskgo]

@strub I notice that you've changed the error messages. I wrote them the way I did (with a single string literal with a \n in the middle) on purpose. The version with two string literals doesn't print properly in my emacs, emitting just the first string.

[strub]

@strub I notice that you've changed the error messages. I wrote them the way I did (with a single string literal with a \n in the middle) on purpose. The version with two string literals doesn't print properly in my emacs, emitting just the first string.

The carriage return should be avoided because you do not know the width of the output display (especially with the VSCode plugin coming in line).

I don't understand what the 2 string literals solution does not work. Will look at it now.

[strub]

@oskgo Fixed in 7367ad0