API error when Datashare schema is not provisioned

**Describe the bug**
API error when **Datashare** schema is not provisioned. Calling the the /accounts resource, the API times out with a 503 error that is not propagated to the client. These are the server logs:
```
2020-05-29 15:26:23.526 EDT(node:1) UnhandledPromiseRejectionWarning: Error: Not found: Dataset agp-dj-001:datashare was not found in location US
2020-05-29 15:26:23.526 EDT at new ApiError (/shared/node_modules/@google-cloud/common/build/src/util.js:58:15)
2020-05-29 15:26:23.526 EDT at /shared/node_modules/@google-cloud/bigquery/build/src/bigquery.js:1066:23
2020-05-29 15:26:23.526 EDT at /shared/node_modules/@google-cloud/common/build/src/util.js:367:25
2020-05-29 15:26:23.526 EDT at Util.handleResp (/shared/node_modules/@google-cloud/common/build/src/util.js:144:9)
2020-05-29 15:26:23.526 EDT at /shared/node_modules/@google-cloud/common/build/src/util.js:432:22
2020-05-29 15:26:23.526 EDT at onResponse (/shared/node_modules/retry-request/index.js:206:7)
2020-05-29 15:26:23.526 EDT at /shared/node_modules/teeny-request/build/src/index.js:233:13
2020-05-29 15:26:23.526 EDT at processTicksAndRejections (internal/process/task_queues.js:85:5)
2020-05-29 15:26:23.526 EDT(node:1) UnhandledPromiseRejectionWarning: Unhandled promise rejection. This error originated either by throwing inside of an async function without a catch block, or by rejecting a promise which was not handled with .catch(). (rejection id: 1)
```

**Version**
[v0.3.0](https://github.com/GoogleCloudPlatform/datashare-toolkit/tree/v0.3.0)

**To Reproduce**
Steps to reproduce the behavior:
1. Deploy API
2. Call /accounts endpoint without provisioning Datashare schema
3. See error

**Expected behavior**
Propagate descriptive error that schema DNE up to client and handle gracefully.