Fix MustCombineARecord error message for //

[sjakobi]

For {=} // 1, we would previously report:

You tried to combine the following value:

↳ {=}

... which is not a record, but is actually a:

↳ 1

Now we report:

You tried to combine the following value:

↳ 1

... which is not a record, but is actually a:

↳ Natural

Fixes #1701.

[sjakobi]

I basically copied this patch from the error handling for /\, which already supplied the correct arguments to MustCombineARecord.