Use I18n for media attachment validation errors

These are currently user facing errors, but are not localized. This adds the
ability for these messages to be localized.

app/services/post_status_service.rb

@@ -37,11 +37,11 @@ class PostStatusService < BaseService
   def validate_media!(media_ids)
     return if media_ids.nil? || !media_ids.is_a?(Enumerable)
 
-    raise Mastodon::ValidationError, 'Cannot attach more than 4 files' if media_ids.size > 4
+    raise Mastodon::ValidationError, I18n.t('media_attachments.validations.too_many') if media_ids.size > 4
 
     media = MediaAttachment.where(status_id: nil).where(id: media_ids.take(4).map(&:to_i))
 
-    raise Mastodon::ValidationError, 'Cannot attach a video to a toot that already contains images' if media.size > 1 && media.find(&:video?)
+    raise Mastodon::ValidationError, I18n.t('media_attachments.validations.images_and_video') if media.size > 1 && media.find(&:video?)
 
     media
   end

config/locales/en.yml

@@ -163,3 +163,7 @@ en:
     invalid_otp_token: Invalid two-factor code
   will_paginate:
     page_gap: "…"
+  media_attachments:
+    validations:
+      too_many: Cannot attach more than 4 files
+      images_and_video: Cannot attach a video to a status that already contains images

spec/services/post_status_service_spec.rb

@@ -141,7 +141,7 @@ RSpec.describe PostStatusService do
       )
     end.to raise_error(
       Mastodon::ValidationError,
-      'Cannot attach more than 4 files',
+      I18n.t('media_attachments.validations.too_many'),
     )
   end
 
@@ -160,7 +160,7 @@ RSpec.describe PostStatusService do
       )
     end.to raise_error(
       Mastodon::ValidationError,
-      'Cannot attach a video to a toot that already contains images',
+      I18n.t('media_attachments.validations.images_and_video'),
     )
   end