Add details to error response for POST /api/v1/accounts in REST API (#15803)

config/locales/activerecord.en.yml

@@ -5,13 +5,28 @@ en:
       poll:
         expires_at: Deadline
         options: Choices
+      user:
+        agreement: Service agreement
+        email: E-mail address
+        locale: Locale
+        password: Password
+      user/account:
+        username: Username
+      user/invite_request:
+        text: Reason
     errors:
       models:
         account:
           attributes:
             username:
-              invalid: only letters, numbers and underscores
+              invalid: must contain only letters, numbers and underscores
+              reserved: is reserved
         status:
           attributes:
             reblog:
               taken: of status already exists
+        user:
+          attributes:
+            email:
+              blocked: uses a disallowed e-mail provider
+              unreachable: does not seem to exist