`backend-*` spring cleaning for erroring code

[simonpcouch]

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Will start out with just one backend as a demo, but imagining one PR per checkbox here (applying to all backends, rather than one PR per backend with all of the following complete in each. Happy to adjust here.):

• [x] #1552
  • ...to snapshot when testing the error message
  • ...to classed error when testing an error from another package or an error thrown from a widely-used helper
  • ...to expect_no_error() (or no test) when regex = NA
  • Analogous story for expect_warning() and expect_message().
• [x] #1555
• [ ] Transition erroring code to cli
  • Update snapshots
• [ ] Test untested erroring code