transition `expect_error()` for snowflake backend

[simonpcouch]

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Related to the first checkbox in #1546 but does not close; a demo of what that PR would look like for all backends.

For snowflake, transition expect_error():

• ...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().