backend-* spring cleaning for erroring code

[simonpcouch]

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.):

• audit calls to expect_error() for backend-*.R files #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().
• Use type checkers #1555
• Transition erroring code to cli
  • Update snapshots
• Test untested erroring code