download/mrc: use cleaner coding style

top-down order for all logic, and shorter code lines,
conforming to rfc 2646 (no more than 80 characters)

the 80-character rule is violated for variables containing
long strings, such as wayback machine urls (can't be helped)

a few bugs were discovered, which will be fixed in follow-up
revisions, such as:

* exit status not handled inside subshell
* in general, exit status should be handled
  more explicitly, rather than relying on -e

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