Put back default strings on dialogs

But make them work by calling _t in render rather than
getDefaultProps().

Also sort out some 'Warning!' strings

src/components/views/dialogs/ErrorDialog.js

@@ -27,6 +27,7 @@ limitations under the License.
 
 import React from 'react';
 import sdk from '../../../index';
+import { _t } from '../../../languageHandler';
 
 export default React.createClass({
     displayName: 'ErrorDialog',
@@ -44,6 +45,9 @@ export default React.createClass({
     getDefaultProps: function() {
         return {
             focus: true,
+            title: null,
+            description: null,
+            button: null,
         };
     },
 
@@ -57,13 +61,13 @@ export default React.createClass({
         const BaseDialog = sdk.getComponent('views.dialogs.BaseDialog');
         return (
             <BaseDialog className="mx_ErrorDialog" onFinished={this.props.onFinished}
-                    title={this.props.title}>
+                    title={this.props.title || _t('Error')}>
                 <div className="mx_Dialog_content">
-                    {this.props.description}
+                    {this.props.description || _t('An error has occurred.')}
                 </div>
                 <div className="mx_Dialog_buttons">
                     <button ref="button" className="mx_Dialog_primary" onClick={this.props.onFinished}>
-                        {this.props.button}
+                        {this.props.button || _t('OK')}
                     </button>
                 </div>
             </BaseDialog>