Handle methods that fail unconditionally in ContractHandler

[msridhar]

Fixes #943

We extend ContractHandler to insert an unconditional throw into the CFG when encountering a method that fails unconditionally according to its @Contract annotation.

[codecov]

Codecov Report

Attention: Patch coverage is 93.33333% with 1 lines in your changes are missing coverage. Please review.

Project coverage is 87.06%. Comparing base (76f0f77) to head (a794043).

❗ Current head a794043 differs from pull request most recent head b48e7a8. Consider uploading reports for the commit b48e7a8 to get more accurate results

Files	Patch %	Lines
...uber/nullaway/dataflow/cfg/NullAwayCFGBuilder.java	88.88%	1 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff              @@
##             master     #946      +/-   ##
============================================
- Coverage     87.13%   87.06%   -0.07%     
+ Complexity     2011     1995      -16     
============================================
  Files            78       77       -1     
  Lines          6513     6456      -57     
  Branches       1264     1253      -11     
============================================
- Hits           5675     5621      -54     
  Misses          422      422              
+ Partials        416      413       -3     

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[yuxincs]

Nice!!