Revisit all error messsages and user strings for proper i18n patterns

E.g.: https://docs.gitlab.com/ee/development/i18n/externalization.html#ruby-files

See:

• !105783 (comment 1378413840)
• !105783 (comment 1378413856)
• !105783 (comment 1378413866)
• !105783 (comment 1378413879)
• !105783 (comment 1378413905)
• !105783 (comment 1378413923)
• !105783 (comment 1378413930)
• !105783 (comment 1378413941)
• !105783 (comment 1378413952)
• !119865 (comment 1382709865)