mgr/dashboard: fix unncessary traceback when bucket not exist

mgr/dashboard: fix unncessary traceback when bucket not exist

UI has an async validator which calls the GET bucket API to make sure
the bucket name doesn't exist, but the proxy
was not properly handling the http_status_codes which results in raising
a massive traceback in logs whenever you type things in the bucket name
field. So handling that gracefully by capturing the proper status codes
for both RequestException and DashboardException

BEFORE
```
File "/usr/share/ceph/mgr/dashboard/services/exception.py", line 47, in dashboard_exception_handler
return handler(*args, **kwargs)
File "/lib/python3.9/site-packages/cherrypy/_cpdispatch.py", line 54, in _call_
return self.callable(*self.args, **self.kwargs)
File "/usr/share/ceph/mgr/dashboard/controllers/_base_controller.py", line 263, in inner
ret = func(*args, **kwargs)
File "/usr/share/ceph/mgr/dashboard/controllers/_rest_controller.py", line 193, in wrapper
return func(*vpath, **params)
File "/usr/share/ceph/mgr/dashboard/controllers/rgw.py", line 357, in get
result = self.proxy(daemon_name, 'GET', 'bucket', {'bucket': bucket})
File "/usr/share/ceph/mgr/dashboard/controllers/rgw.py", line 213, in proxy
raise DashboardException(e, http_status_code=http_status_code, component='rgw')
dashboard.exceptions.DashboardException: RGW REST API failed request with status code 404
(b'{"Code":"NoSuchBucket","Message":"","RequestId":"tx00000f14e08c1af0d5615-006'
b'71f54a7-3bc6-default","HostId":"3bc6-default-default"}')
2024-10-28T09:08:55.990+0000 7f89e045b640  0 [dashboard INFO request] [::ffff:10.74.18.122:51853] [GET] [500] [0.007s] [admin] [200.0B] /api/rgw/bucket/bucket-das
```

AFTER
```
Jul 08 09:28:28 ceph-node-00 ceph-mgr[2243]: [dashboard ERROR dashboard.rest_client] RGW REST API failed GET req status: 404
Jul 08 09:28:28 ceph-node-00 ceph-mgr[2243]: [dashboard INFO dashboard.services.exception] Dashboard Exception: RGW REST API failed request with status code 404
                                             (b'{"Code":"NoSuchBucket","Message":"","RequestId":"tx00000c029a81e5d154844-006'
                                              b'a4e183c-14251-default","HostId":"14251-default-default"}')
Jul 08 09:28:28 ceph-node-00 ceph-mgr[2243]: [dashboard INFO dashboard.tools] [::ffff:192.168.100.1:60408] [GET] [404] [0.078s] [admin] [200.0B] /api/rgw/bucket/testsa
```

Fixes: https://tracker.ceph.com/issues/78038
Signed-off-by: Nizamudeen A <nia@redhat.com>