throw 404 exception when get non-existing asset

[yangyangv2]

Summary

When ./getAsset on non-existing asset, it should throw 404 exception

Testing Done

unit test

Checklist

• The PR conforms to DataHub's Contributing Guideline (particularly Commit Message Format)
• Links to related issues (if applicable)
• Docs related to the changes have been added/updated (if applicable)

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 66.16%. Comparing base (8ccc25e) to head (d4d9cef).
Report is 1 commits behind head on master.

Additional details and impacted files

@@             Coverage Diff              @@
##             master     #480      +/-   ##
============================================
- Coverage     66.35%   66.16%   -0.19%     
- Complexity     1523     1527       +4     
============================================
  Files           137      137              
  Lines          5849     5891      +42     
  Branches        642      650       +8     
============================================
+ Hits           3881     3898      +17     
- Misses         1693     1713      +20     
- Partials        275      280       +5     

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[jphui]

lgtm