Switch from expect to map_err.

Switch from expect to map_err.

This lets us make a clean exit rather than triggering a panic, (which
has some extra noise and suggests something more exceptional than we
really want here).

Note our message is reworded slightly since map_err doesn't give us
full control of the string. It emits:

	Error: "<our-string>"

so we don't want to start our string with "Error" as that would look
redundant.

I don't love not having full control of the error message, but still,
I think it is a reasonable compromise given how clean the code is with
.map_err.