Address comments from PR #8418.
auto merge of #8467 : pnkfelix/rust/fsk-followup-pr8418, r=pcwalton

Comments addressed were:

 * nit: we prefer "expected foo, found bar" to "expected foo but found bar"

 * nit: lowercase initial letter of error message
   * (i fixed other instances of this in the file that were not part of the original PR)

 * "just do let _ = s;" (for suppressing unused variable warning).