Handle Horizon Service Unavailable Errors

[sentry-io]

Sentry Issue: STELLAR-EXPLORER-31

Error: Service Unavailable
  File "/app/node_modules/stellar-sdk/src/errors.ts", line 17, in new NetworkError
  File "/app/node_modules/stellar-sdk/src/call_builder.ts", line 417, in <anonymous>
  File "/app/node_modules/stellar-sdk/node_modules/tslib/tslib.js", line 141, in step
    op = body.call(thisArg, _);
  File "/app/node_modules/stellar-sdk/node_modules/tslib/tslib.js", line 122, in Object.next
    function verb(n) { return function (v) { return step([n, v]); }; }
  File "/app/node_modules/stellar-sdk/node_modules/tslib/tslib.js", line 115, in <anonymous>
    step((generator = generator.apply(thisArg, _arguments || [])).next());
...
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