Cleanup ImpersonatedCredentials

[chingor13]

• Only use a single private constructor
• Caught and wrapped exceptions now provide better message and set the original error as the cause
• Various documentation fixes

[codecov-io]

Codecov Report

❗ No coverage uploaded for pull request base (master@b037146). Click here to learn what that means.
The diff coverage is 91.17%.

@@            Coverage Diff            @@
##             master     #212   +/-   ##
=========================================
  Coverage          ?   76.86%           
  Complexity        ?      327           
=========================================
  Files             ?       21           
  Lines             ?     1465           
  Branches          ?      158           
=========================================
  Hits              ?     1126           
  Misses            ?      260           
  Partials          ?       79

Impacted Files	Coverage Δ	Complexity Δ
...om/google/auth/oauth2/ImpersonatedCredentials.java	84.54% <91.17%> (ø)	11 <7> (?)

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