Use full error messages for relations validation.

app/assets/javascripts/angular/helpers/components/api-helper.js

@@ -38,11 +38,12 @@ angular.module('openproject.workPackages.helpers')
     },
 
     getErrorMessage: function(error) {
-      var response = JSON.parse(error.responseText);
       if(error.status == 422) {
-        return response.description + response.errors.map(function(error) {
-          return " \"" + error.key + "\"" + " " + error.messages[0];
-        }).join('. ');
+        var response = JSON.parse(error.responseText);
+        return response.errors.join('. ');
+      }
+      if(error.status == 500) {
+        return error.statusText;
       }
     },
 

lib/api/errors/validation.rb

@@ -41,7 +41,7 @@ module API
       end
 
       def errors
-        @obj.errors.messages.map{ |m| { key: m[0], messages: m[1] }}
+        @obj.errors.full_messages
       end
 
       def to_json

lib/api/v3/relations/relations_api.rb

@@ -25,7 +25,7 @@ module API
               relation.save!
               representer.to_json
             else
-              raise ::API::Errors::Validation.new(relation)
+              fail Errors::Validation.new(relation)
             end
           end