fix: throw on invalid credentials

[yoshi-automation]

This PR was generated using Autosynth. 🌈

Here's the log from Synthtool:

synthtool > Executing /tmpfs/src/git/autosynth/working_repo/synth.py.
synthtool > Ensuring dependencies.
synthtool > Pulling artman image.
latest: Pulling from googleapis/artman
Digest: sha256:5fd9aee1d82a00cebf425c8fa431f5457539562f5867ad9c54370f0ec9a7ccaa
Status: Image is up to date for googleapis/artman:latest
synthtool > Cloning googleapis.
synthtool > Running generator for google/datastore/artman_datastore.yaml.
synthtool > Generated code into /home/kbuilder/.cache/synthtool/googleapis/artman-genfiles/js/datastore-v1.
synthtool > Replaced '../../package.json' in src/v1/datastore_client.js.
.eslintignore
.eslintrc.yml
.github/ISSUE_TEMPLATE/bug_report.md
.github/ISSUE_TEMPLATE/feature_request.md
.github/ISSUE_TEMPLATE/support_request.md
.jsdoc.js
.kokoro/common.cfg
.kokoro/continuous/node10/common.cfg
.kokoro/continuous/node10/test.cfg
.kokoro/continuous/node11/common.cfg
.kokoro/continuous/node11/test.cfg
.kokoro/continuous/node6/common.cfg
.kokoro/continuous/node6/test.cfg
.kokoro/continuous/node8/common.cfg
.kokoro/continuous/node8/docs.cfg
.kokoro/continuous/node8/lint.cfg
.kokoro/continuous/node8/samples-test.cfg
.kokoro/continuous/node8/system-test-grpcjs.cfg
.kokoro/continuous/node8/system-test.cfg
.kokoro/continuous/node8/test.cfg
.kokoro/docs.sh
.kokoro/lint.sh
.kokoro/presubmit/node10/common.cfg
.kokoro/presubmit/node10/test.cfg
.kokoro/presubmit/node11/common.cfg
.kokoro/presubmit/node11/test.cfg
.kokoro/presubmit/node6/common.cfg
.kokoro/presubmit/node6/test.cfg
.kokoro/presubmit/node8/common.cfg
.kokoro/presubmit/node8/docs.cfg
.kokoro/presubmit/node8/lint.cfg
.kokoro/presubmit/node8/samples-test.cfg
.kokoro/presubmit/node8/system-test-grpcjs.cfg
.kokoro/presubmit/node8/system-test.cfg
.kokoro/presubmit/node8/test.cfg
.kokoro/presubmit/windows/common.cfg
.kokoro/presubmit/windows/test.cfg
.kokoro/publish.sh
.kokoro/release/publish.cfg
.kokoro/samples-test.sh
.kokoro/system-test.sh
.kokoro/test.bat
.kokoro/test.sh
.kokoro/trampoline.sh
.nycrc
.prettierignore
.prettierrc
CODE_OF_CONDUCT.md
CONTRIBUTING.md
LICENSE
codecov.yaml
renovate.json
npm WARN deprecated text-encoding@0.6.4: no longer maintained
npm WARN deprecated calmcard@0.1.1: no longer maintained
npm WARN deprecated nopter@0.3.0: try optionator
npm WARN deprecated urlobj@0.0.11: use universal-url, minurl, relateurl, url-relation
npm WARN deprecated circular-json@0.3.3: CircularJSON is in maintenance only, flatted is its successor.

> grpc@1.18.0 install /tmpfs/src/git/autosynth/working_repo/node_modules/grpc
> node-pre-gyp install --fallback-to-build --library=static_library

node-pre-gyp WARN Using needle for node-pre-gyp https download 
[grpc] Success: "/tmpfs/src/git/autosynth/working_repo/node_modules/grpc/src/node/extension_binary/node-v57-linux-x64-glibc/grpc_node.node" is installed via remote

> protobufjs@6.8.8 postinstall /tmpfs/src/git/autosynth/working_repo/node_modules/protobufjs
> node scripts/postinstall


> @google-cloud/datastore@3.1.0 prepare /tmpfs/src/git/autosynth/working_repo
> npm run compile


> @google-cloud/datastore@3.1.0 compile /tmpfs/src/git/autosynth/working_repo
> tsc -p . && cp -r src/v1 build/src && cp -r protos build && cp test/*.js build/test

npm notice created a lockfile as package-lock.json. You should commit this file.
npm WARN optional SKIPPING OPTIONAL DEPENDENCY: fsevents@1.2.7 (node_modules/fsevents):
npm WARN notsup SKIPPING OPTIONAL DEPENDENCY: Unsupported platform for fsevents@1.2.7: wanted {"os":"darwin","arch":"any"} (current: {"os":"linux","arch":"x64"})

added 1481 packages from 1346 contributors and audited 5313 packages in 36.763s
found 4 vulnerabilities (3 low, 1 high)
  run `npm audit fix` to fix them, or `npm audit` for details

> @google-cloud/datastore@3.1.0 fix /tmpfs/src/git/autosynth/working_repo
> gts fix && eslint '**/*.js' --fix


/tmpfs/src/git/autosynth/working_repo/samples/concepts.js
  25:29  error  "@google-cloud/datastore" is not found  node/no-missing-require

/tmpfs/src/git/autosynth/working_repo/samples/quickstart.js
  20:29  error  "@google-cloud/datastore" is not found  node/no-missing-require

/tmpfs/src/git/autosynth/working_repo/samples/system-test/tasks.test.js
  18:29  error  "@google-cloud/datastore" is not found  node/no-missing-require

/tmpfs/src/git/autosynth/working_repo/samples/tasks.js
  23:29  error  "@google-cloud/datastore" is not found  node/no-missing-require

✖ 4 problems (4 errors, 0 warnings)

npm ERR! code ELIFECYCLE
npm ERR! errno 1
npm ERR! @google-cloud/datastore@3.1.0 fix: `gts fix && eslint '**/*.js' --fix`
npm ERR! Exit status 1
npm ERR! 
npm ERR! Failed at the @google-cloud/datastore@3.1.0 fix script.
npm ERR! This is probably not a problem with npm. There is likely additional logging output above.

npm ERR! A complete log of this run can be found in:
npm ERR!     /home/kbuilder/.npm/_logs/2019-02-13T12_12_48_765Z-debug.log
synthtool > Cleaned up 2 temporary directories.
synthtool > Wrote metadata to synth.metadata.

[codecov]

Codecov Report

Merging #331 into master will decrease coverage by 0.33%.
The diff coverage is 0%.

@@            Coverage Diff             @@
##           master     #331      +/-   ##
==========================================
- Coverage   97.93%   97.59%   -0.34%     
==========================================
  Files           7        7              
  Lines         581      583       +2     
  Branches       63       63              
==========================================
  Hits          569      569              
- Misses          4        6       +2     
  Partials        8        8

Impacted Files	Coverage Δ
src/v1/datastore_client.js	90.76% <0%> (-2.89%)	⬇️

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