Change exceptions used to catch failed GCE authentication

[mxr576]

Closes #333

[codecov]

Codecov Report

Merging #334 (7577a3f) into 3.x (a882b50) will not change coverage.
The diff coverage is 0.00%.

Additional details and impacted files

@@            Coverage Diff            @@
##                3.x     #334   +/-   ##
=========================================
  Coverage     87.60%   87.60%           
  Complexity     1750     1750           
=========================================
  Files           349      349           
  Lines          4704     4704           
=========================================
  Hits           4121     4121           
  Misses          583      583           

Flag	Coverage Δ
unittests	87.60% <0.00%> (ø)

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Files	Coverage Δ
...Client/Plugin/Authentication/GceServiceAccount.php	0.00% <0.00%> (ø)

[mxr576]

Alternative considering, but it feels an overkill to me, catch \Psr\Http\Client\ClientExceptionInterface

Maybe this approach combines best from both worlds, isAvailable() should only fail with NetworkExceptionInterface, when the project is not hosted on GCE -- right? However getAccessToken() can also fail when the service account is not configured properly. so the host was reachable and it returned an HTTP >= 400 code.

[shishir-intelli]

LGTM