Uses rpc's client_error::Error::kind() in more places

[brooksprumo]

Problem

If/when we change rpc's client_error::Error::kind field from ErrorKind to Box<ErrorKind>, there are miscellaneous uses of kind that need to be updated (as expected). It turns out that many uses of kind actually want a reference, i.e. &ErrorKind. There's also already a function on client_error::Error that does just this: Error::kind(). So we can/should use that function instead, which means fewer changes later when modifying the underlying kind type.

Please see #6290 (comment) for the full context.

Summary of Changes

Use kind() in more places.

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 25.00000% with 3 lines in your changes missing coverage. Please review.

Project coverage is 82.7%. Comparing base (640a749) to head (f481ef5).

Additional details and impacted files

@@           Coverage Diff            @@
##           master    #6292    +/-   ##
========================================
  Coverage    82.7%    82.7%            
========================================
  Files         845      845            
  Lines      377813   377813            
========================================
+ Hits       312659   312814   +155     
+ Misses      65154    64999   -155     

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