Per #260, add logging when a domain is passed that isn't found in the metadata

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Closes #260

The scope of {xportr}

{xportr}'s scope is to enable R users to write out submission compliant xpt files that can be delivered to a Health Authority or to downstream validation software programs. We see labels, lengths, types, ordering and formats from a dataset specification object (SDTM and ADaM) as being our primary focus. We also see messaging and warnings to users around applying information from the specification file as a primary focus. Please make sure your Pull Request meets this scope of {xportr}. If your Pull Request moves beyond this scope, please get in touch with the {xportr} team on slack or create an issue to discuss.

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Task List

• The spirit of xportr is met in your Pull Request
• Place Closes #<insert_issue_number> into the beginning of your Pull Request Title (Use Edit button in top-right if you need to update)
• Summary of changes filled out in the above Changes Description. Can be removed or left blank if changes are minor/self-explanatory.
• Code is formatted according to the tidyverse style guide. Use styler package and functions to style files accordingly.
• New functions or arguments follow established convention found in the Wiki.
• Updated relevant unit tests or have written new unit tests. See our Wiki for conventions used in this package.
• Creation/updated relevant roxygen headers and examples. See our Wiki for conventions used in this package.
• Run devtools::document() so all .Rd files in the man folder and the NAMESPACE file in the project root are updated appropriately
• Run pkgdown::build_site() and check that all affected examples are displayed correctly and that all new/updated functions occur on the "Reference" page.
• Update NEWS.md if the changes pertain to a user-facing function (i.e. it has an @export tag) or documentation aimed at users (rather than developers)
• The NEWS.md entry should go under the # xportr development version section. Don't worry about updating the version because it will be auto-updated using the vbump.yaml CI.
• Address any updates needed for vignettes and/or templates.
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• The developer is responsible for fixing merge conflicts not the Reviewer.
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