fix(gcs): stop handling DefaultCredentialsError

[justindujardin]

• google-cloud-storage provides a nice error message when it can't find the default credentials. Let it flow through to the client.
• add test to ensure that if the default google creds can't be found, the error can be caught by the caller.
• fixes #43

[codecov[bot]]

Codecov Report

Merging #50 (d4754ff) into develop (c98df80) will increase coverage by 0.37%. The diff coverage is 100.00%.

@@             Coverage Diff             @@
##           develop      #50      +/-   ##
===========================================
+ Coverage    93.52%   93.90%   +0.37%     
===========================================
  Files           12       13       +1     
  Lines         1777     1869      +92     
===========================================
+ Hits          1662     1755      +93     
+ Misses         115      114       -1     

Impacted Files	Coverage Δ
pathy/__init__.py	100.00% <100.00%> (ø)
pathy/gcs.py	80.50% <100.00%> (+0.62%)	:arrow_up:
pathy/tests/test_gcs.py	100.00% <100.00%> (ø)
pathy/tests/test_cli.py	100.00% <0.00%> (ø)
pathy/tests/test_base.py	98.39% <0.00%> (+0.03%)	:arrow_up:
pathy/base.py	91.61% <0.00%> (+0.36%)	:arrow_up:
pathy/cli.py	91.30% <0.00%> (+1.43%)	:arrow_up:

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[justindujardin]

:tada: This PR is included in version 0.4.0 :tada:

The release is available on GitHub release

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