bug(auth): Fixing missing email templates for production build

[dschom]

Because

• Email sent using the new email library were failing.

This pull request

• Resolves an issue where the templates assets (.mjml & .txt) files were getting copied into the dist folder.
• Provides a more developer friendly error message if the asset cannot be loaded.

Issue that this pull request solves

Closes: (issue number)

Checklist

Put an x in the boxes that apply

• My commit is GPG signed.
• If applicable, I have modified or added tests which pass locally.
• I have added necessary documentation (if appropriate).
• I have verified that my changes render correctly in RTL (if appropriate).

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Before:

After:

Other information (Optional)

I feel like there should be a better way to do this, but... this will do for now. As mentioned in the code comment, the fxa/dist/lib/accounts/email-render/* is correct and has the extra assets. It'd be nice if all our builds were housed in the fxa/dist folder (ie mono repo root), butt that's too big of a change for a patch.