feat: Raise GoogleAPICallError on REST response errors

[vchudnov-g]

While testing against Showcase, I found that server errors resulted in a large exception stack dump that did not include the body of the erroring response. This PR shortens the stack dump and prints out the request body.

Fixes #769

[codecov]

Codecov Report

Merging #891 (296d8e0) into master (7c185e8) will not change coverage.
The diff coverage is 100.00%.

@@            Coverage Diff            @@
##            master      #891   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files           26        27    +1     
  Lines         1608      1697   +89     
  Branches       328       347   +19     
=========================================
+ Hits          1608      1697   +89     

Impacted Files	Coverage Δ
gapic/samplegen_utils/types.py	100.00% <ø> (ø)
gapic/samplegen_utils/utils.py	100.00% <ø> (ø)
gapic/utils/case.py	100.00% <ø> (ø)
gapic/utils/reserved_names.py	100.00% <ø> (ø)
gapic/generator/generator.py	100.00% <100.00%> (ø)
gapic/samplegen/samplegen.py	100.00% <100.00%> (ø)
gapic/schema/api.py	100.00% <100.00%> (ø)
gapic/schema/metadata.py	100.00% <100.00%> (ø)
gapic/schema/wrappers.py	100.00% <100.00%> (ø)
gapic/utils/__init__.py	100.00% <100.00%> (ø)
... and 3 more

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[busunkim96]

I think google-api-core has some functionality for handling error messages. Could that be used instead? See the original feature request in #769

[vchudnov-g]

Oh, interesting. Thanks for the pointers! Made changes; PTAL.

[vchudnov-g]

Also changed PR title and description to match.