fix: Return error if subscription name already exists.

[stephenplusplus]

Fixes #122

This could be a breaking change, depending on if you consider this a bug that we fixed or a behavior that we switched.

Before:

topic.createSubscription('my-subscription', function(err, subscription) {})

// in the future...
topic.createSubscription('my-subscription', function(err, subscription) {})

After:

topic.createSubscription('my-subscription', function(err, subscription) {})

// in the future...
topic.createSubscription('my-subscription', function(err, subscription) {
  // New error returned saying you already have a subscription with this name!
})

Here's a whole talk about some of the history: googleapis/google-cloud-node#1257

[JustinBeckwith]

I think we're doing a semver major release at some point soon anyway :)

[codecov]

Codecov Report

Merging #226 into master will not change coverage.
The diff coverage is 100%.

@@          Coverage Diff          @@
##           master   #226   +/-   ##
=====================================
  Coverage     100%   100%           
=====================================
  Files          10     10           
  Lines         807    807           
=====================================
  Hits          807    807

Impacted Files	Coverage Δ
src/index.js	100% <100%> (ø)	⬆️

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[JustinBeckwith]

@kinwa91 lets get this in 0.20 too.

[art-pumpkin]

@stephenplusplus @JustinBeckwith this was a breaking change for us, using the google bookshelf boilerplate...
What is the recommended workaround for getting the old behavior back? Could you please provide some insight? Thanks!

FYI for those experiencing the following error:
Error: 6 ALREADY_EXISTS: Resource already exists in the project (resource=YOUR_SUB_NAME)

I used the following snippet for promises-style. I needed to differentiate between a createSubscription object and a subscription object.

let subscriptionObject = await (async (topic, subscriptionName) => { try { return (await topic.createSubscription(subscriptionName))[0]; } catch (err) { if (err.code === 6) { logging.debug('Subscription Exists'); return await topic.subscription(subscriptionName); } else throw err; } })(topic, subscriptionName);

[stephenplusplus]

You can do:

topic.createSubscription('my-subscription', function(err, subscription) {})

// in the future...
const subscription = topic.subscription('my-subscription')

subscription.get({ autoCreate: true }, function(err) {
  // No more `ALREADY_EXISTS` error!
})

Alternatively, if you know the subscription exists, you can just start using it right away:

const subscription = topic.subscription('my-subscription')

subscription.on('message', msg => {})