Follow-up to #50104.

The error conditions are that the adapter is spelled incorrectly __or__
the necessary gem isn't in the `Gemfile`.  Therefore, to fix the error,
the user must ensure that the adapter is spelled correctly __and__ the
gem is in the `Gemfile`.

This commit also adds `require "active_support/core_ext/string/filters"`,
which technically should be included for `String#squish`.