(html) Clarify warning in mail composer

UI/MailerUI/English.lproj/Localizable.strings

@@ -321,6 +321,7 @@
 = "The messages could not be moved to the trash folder. Would you like to delete them immediately?";
 
 /* Message editing */
+"Please specify a subject." = "Please specify a subject.";
 "error_missingsubject" = "The message has no subject. Are you sure you want to send it?";
 "error_missingrecipients" = "Please specify at least one recipient.";
 "Send Anyway" = "Send Anyway";

UI/Templates/MailerUI/UIxMailEditor.wox

@@ -167,7 +167,7 @@
               </label>
               <input type="text" name="subject" ng-model="editor.message.editable.subject" ng-required="true"/>
               <div ng-messages="messageForm.subject.$error">
-                <div ng-message="required"><var:string label:value="error_missingsubject"/></div>
+                <div ng-message="required"><var:string label:value="Please specify a subject."/></div>
               </div>
             </md-input-container>
           </div>