[shared file view] code improvement (#3185)

* added component shared-file-view.js & shared-file-view-tip.js
* modified 'text file view'

frontend/src/components/shared-file-view/shared-file-view-tip.js

@@ -0,0 +1,24 @@
+import React from 'react';
+import { gettext } from '../../utils/constants';
+
+const { err } = window.shared.pageOptions;
+
+class SharedFileViewTip extends React.Component {
+  render() {
+    let errorMsg;
+    if (err == 'File preview unsupported') {
+      errorMsg = <p>{gettext('Online view is not applicable to this file format')}</p>;
+    } else {
+      errorMsg = <p className="error">{err}</p>;
+    }
+    return (
+      <div className="shared-file-view-body">
+        <div className="file-view-tip">
+          {errorMsg}
+        </div>
+      </div>
+    );
+  }
+}
+
+export default SharedFileViewTip;