[JENKINS-52869] Show error messages on user creation (#3727)

* [JENKINS-52869] Show error messages on user creation

* Restore support for general signup error

(cherry picked from commit 3b8258fd)

core/src/main/java/hudson/security/HudsonPrivateSecurityRealm.java

@@ -392,7 +392,7 @@ public class HudsonPrivateSecurityRealm extends AbstractPasswordBasedSecurityRea
      * @param validateCaptcha  whether to attempt to validate a captcha in the request
      *
      * @return a {@link SignupInfo#SignupInfo(StaplerRequest) SignupInfo from given request}, with {@link
-     * SignupInfo#errors} set to a non-null value if any of the supported fields are invalid
+     * SignupInfo#errors} containing errors (keyed by field name), if any of the supported fields are invalid
      */
     private SignupInfo validateAccountCreationForm(StaplerRequest req, boolean validateCaptcha) {
         // form field validation
@@ -554,10 +554,16 @@ public class HudsonPrivateSecurityRealm extends AbstractPasswordBasedSecurityRea
         public String username,password1,password2,fullname,email,captcha;
 
         /**
-         * To display an error message, set it here.
+         * To display a general error message, set it here.
+         *
          */
         public String errorMessage;
 
+        /**
+         * Add field-specific error messages here.
+         * Keys are field names (e.g. {@code password2}), values are the messages.
+         */
+        // TODO i18n?
         public HashMap<String, String> errors = new HashMap<String, String>();
 
         public SignupInfo() {

core/src/main/resources/hudson/security/HudsonPrivateSecurityRealm/_entryForm.jelly

@@ -32,6 +32,11 @@ THE SOFTWARE.
         ${data.errorMessage}
       </div>
     </j:if>
+    <j:forEach var="error" items="${data.errors}">
+      <div class="error" style="margin-bottom:1em">
+        ${error.value}
+      </div>
+    </j:forEach>
       <table>
         <tr>
           <td>${%Username}:</td>