throw exception instead of fall back to default credentials chain

[JacksonTian]

You need to complete

• unit tests and/or feature tests
• documentation is changed or added

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 97.33333% with 2 lines in your changes missing coverage. Please review.

Project coverage is 91.37%. Comparing base (6ae5ad8) to head (f5789e5).
Report is 2 commits behind head on master.

Files with missing lines	Patch %	Lines
...c/main/java/com/aliyun/credentials/Credential.java	66.66%	1 Missing and 1 partial ⚠️

Additional details and impacted files

@@             Coverage Diff              @@
##             master      #92      +/-   ##
============================================
- Coverage     91.39%   91.37%   -0.03%     
  Complexity      613      613              
============================================
  Files            45       45              
  Lines          2208     2202       -6     
  Branches        169      169              
============================================
- Hits           2018     2012       -6     
  Misses          128      128              
  Partials         62       62              

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