Add an errorHandler property to constructor options

[ghost]

✔️ All existing tests pass
✔️ New functionality is unit-tested

Addresses #320 #326

What this adds

This PR adds an optional errorHandler property to the class constructor's options.
If unspecified, the behaviour of the library does not change.
If specified, when an axios request fails, errorHandler(axiosError) will be called instead of re-throwing the axios error.

Example usage:

const Analytics = require('analytics-node');

const client = new Analytics('write key', {
  errorHandler: (err) => {
    console.error('analytics-node flush failed.')
    console.error(err)
  }
});

client.track({
  event: 'event name',
  userId: 'user id'
});
// ☝️ if this fails when flushed no exception will be thrown and instead
// the axios error will be logged to the console.

Why

Since no way to handle the re-thrown axios error was exposed previously, this meant that if the axios request failed for any reason, the application using analytics-node would crash. A workaround was to use node's process unhandledRejection or unhandledException handlers, but this is very un-ergonomic for library users.

Work remaining

This functionality needs to be documented: https://segment.com/docs/connections/sources/catalog/libraries/server/node/

[pooyaj]

Looks great 🙌

[nd4p90x]

PR For Node Documentation Update:
segmentio/segment-docs#3111