Add "Close" button in Brave Rewards wallet verification error modal

[Miyayes]

Description

In our Brave Rewards error modal (particularly when wallet verification fails), no "close" button is present. The user can only "Try again", in which case they are sent into a loop (unless they close out of the tab).

Solution

Add a close button that dismisses the modal. Update: Specifically, add an x button in the top right corner, per design team.

[Miyayes]

Design team (Cody) says to just add an x in the top right corner.

[stephendonner]

Verified PASSED using the steps and testplan from #15264

Brave	1.25.46 Chromium: 90.0.4430.93 (Official Build) nightly (x86_64)
Revision	4df112c29cfe9a2c69b14195c0275faed4e997a7-refs/branch-heads/4430@{#1348}
OS	macOS Version 11.3 (Build 20E232)

Steps:

1. set my macOS region to Japan, language to Japanese, restarted the OS
2. launched Brave and enabled Rewards via the NTP page
3. relaunched Brave using --enable-logging=stderr --vmodule="*/bat-native-ledger/*"=6,"*/brave_rewards/*"=6,"*/bat-native-ads/*"=6,"*/bat-native-confirmations/*"=6,"*/brave_ads/*"=9,"*/brave_user_model/*"=6 --brave-ads-staging --brave-ads-debug --rewards=staging=true,reconcile-interval=4
4. via brave://flags, set Enable bitFlyer for Brave Rewards to Enabled, and restarted using the commandline, above
5. clicked on the Japanese-text equivalent of Verify Wallet
6. confirmed I was taken to the bitFlyer staging environment
7. clicked on the blue/purple button (not pictured, sorry!)
8. received an error message
9. confirmed that there was a (x) widget at the top right, and that clicking on it closed the error-message popup, and returned me to brave://rewards

example	example	example	example	example

[LaurenWags]

Close button is present, but is not working - see #15963 (comment)

Brave | 1.25.64 Chromium: 90.0.4430.212 (Official Build) beta （x86_64）
-- | --
変更履歴 | e3cd97fc771b893b7fd1879196d1215b622c2bed-refs/branch-heads/4430@{#1429}
OS | macOS バージョン10.15.7（ビルド19H1030）