test: convert a portion of tests from easymock to mockito

[dmitry-fa]

part of #270

• mokito tests made stronger: mocks throw an exception if not redefined explicitly
• added missed tests for cases when RPC causes an exception
• Storage reader()/writer() tests converted from easymock to mockito

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[codecov]

Codecov Report

Merging #330 into master will increase coverage by 0.07%.
The diff coverage is n/a.

@@             Coverage Diff              @@
##             master     #330      +/-   ##
============================================
+ Coverage     62.63%   62.71%   +0.07%     
- Complexity      553      554       +1     
============================================
  Files            31       31              
  Lines          5023     5023              
  Branches        480      480              
============================================
+ Hits           3146     3150       +4     
+ Misses         1711     1708       -3     
+ Partials        166      165       -1     

Impacted Files	Coverage Δ	Complexity Δ
...ain/java/com/google/cloud/storage/StorageImpl.java	80.40% <0.00%> (+0.12%)	131.00% <0.00%> (ø%)
...java/com/google/cloud/storage/BlobReadChannel.java	88.23% <0.00%> (+2.52%)	17.00% <0.00%> (+1.00%)

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