Translate push validation error title

[pglombardo]

Summary

• Wraps the push validation error title in gettext n_() so the full sentence can be translated (including plural forms)
• Fixes [Bug]: Error title is not translated correctly #4447 where French (and other locales) showed an untranslated "1 error prohibited this push from being saved:" while the error details were already translated

Fixes #4447

Test plan

• Visit /p/new?tab=url&locale=fr and submit an invalid URL
• Confirm the validation detail message still appears
• After translation sync, confirm the error title translates (English remains: "1 error prohibited this push from being saved:")
• bin/ci passes