fix(style): Fix the error message/heading styles in settings (#6216) r=@philbooth
fix(style): Fix the error message/heading styles in settings (#6216) r=@philbooth

* Remove the bottom-margin of the headings in settings-panels.
* Ensure the error message clears the heading floated content.
* Remove the padding on the error content, that's handled universally.

fixes #6206
mozilla/fxa-content-server,shane-tomlinson/fxa-content-server,shane-tomlinson/fxa-content-server,mozilla/fxa-content-server,mozilla/fxa-content-server,shane-tomlinson/fxa-content-server