Pub/Sub: more refactoring

[sduskis]

• Using Preconditions
• Adding onSuccess and onFailure to OutstandingPublish

[chingor13]

Looks like you need to run the code formatter again.

[codecov]

Codecov Report

Merging #5044 into master will decrease coverage by 0.64%.
The diff coverage is 70.58%.

@@             Coverage Diff              @@
##             master    #5044      +/-   ##
============================================
- Coverage     50.35%   49.71%   -0.65%     
+ Complexity    23685    22317    -1368     
============================================
  Files          2242     2242              
  Lines        226183   221462    -4721     
  Branches      24961    24081     -880     
============================================
- Hits         113888   110091    -3797     
+ Misses       103693   102912     -781     
+ Partials       8602     8459     -143

Impacted Files	Coverage Δ	Complexity Δ
...ain/java/com/google/cloud/pubsub/v1/Publisher.java	88.98% <70.58%> (+2.34%)	23 <6> (ø)	⬇️
...in/java/com/google/cloud/storage/StorageBatch.java	88% <0%> (-4%)	13% <0%> (ø)
...e/cloud/vision/v1p4beta1/ImageAnnotatorClient.java	58.69% <0%> (-3.31%)	15% <0%> (-4%)
...m/google/cloud/vision/v1/ImageAnnotatorClient.java	47.05% <0%> (-2.95%)	9% <0%> (-2%)
...e/cloud/vision/v1p3beta1/ImageAnnotatorClient.java	47.05% <0%> (-2.95%)	9% <0%> (-2%)
...e/cloud/vision/v1p2beta1/ImageAnnotatorClient.java	47.05% <0%> (-2.95%)	9% <0%> (-2%)
.../java/com/google/cloud/speech/v1/SpeechClient.java	48.57% <0%> (-2.78%)	10% <0%> (-2%)
...om/google/cloud/speech/v1p1beta1/SpeechClient.java	48.57% <0%> (-2.78%)	10% <0%> (-2%)
...d/webrisk/v1beta1/WebRiskServiceV1Beta1Client.java	63.41% <0%> (-2.5%)	12% <0%> (-3%)
...ogle/cloud/texttospeech/v1/TextToSpeechClient.java	55.88% <0%> (-2.46%)	9% <0%> (-2%)
... and 543 more

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