chore(controller): return 502 instead of 500 when online eval service is unavailable

[jialeicui]

Description

curl -i http://jialei.pre.intra.starwhale.ai/gateway/model-serving/21/
HTTP/1.1 502
Date: Thu, 29 Dec 2022 08:52:10 GMT
Content-Length: 0
Connection: keep-alive

Modules

• UI
• Controller
• Agent
• Client
• Python-SDK
• Others

Checklist

• run code format and lint check
• add unit test
• add necessary doc

[codecov]

Codecov Report

Merging #1681 (f6c0a66) into main (ed979e8) will decrease coverage by 7.64%.
The diff coverage is 60.00%.

@@             Coverage Diff              @@
##               main    #1681      +/-   ##
============================================
- Coverage     82.99%   75.34%   -7.65%     
- Complexity     1922     1923       +1     
============================================
  Files           350      270      -80     
  Lines         18456     8801    -9655     
  Branches        969      969              
============================================
- Hits          15317     6631    -8686     
+ Misses         2735     1767     -968     
+ Partials        404      403       -1     

Flag	Coverage Δ
controller	75.34% <60.00%> (+0.01%)	⬆️
standalone	?
unittests	?

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
...n/java/ai/starwhale/mlops/common/ProxyServlet.java	67.85% <60.00%> (-1.96%)	⬇️
client/starwhale/core/dataset/tabular.py
client/starwhale/utils/fs.py
client/starwhale/core/model/default_handler.py
client/starwhale/base/bundle_copy.py
client/starwhale/core/model/copy.py
client/starwhale/base/cloud.py
client/starwhale/api/_impl/job.py
client/starwhale/api/_impl/wrapper.py
client/starwhale/api/_impl/dataset/builder.py
... and 73 more

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[anda-ren]

LGTM