firestore.enablePersistence().catch is not catching exceptions

[jamesdaniels]

[REQUIRED] Describe your environment

• Operating System version: any
• Firebase SDK version: <= 4.12.1
• Firebase Product: firestore

Any exceptions with enable persistence raise an uncaught exception, even if you attempt to catch on the Promise.

Steps to reproduce:

Call in two tabs:

firestore.enablePersistence().then(() => {}, () => {})
or firestore.enablePersistence().catch(() => {})

You'll get an uncaught exception RE Firestore persistence already having been initialized.

The exception should have be caught with the Promise's reject argument / catch method and not bubble up further, which raises an uncaught exception. Developers currently need to wrap in a try/catch which isn't great ergo.

This hits us here: https://github.com/angular/angularfire2/blob/e343f137bd0ca9a1d958253b58104c64acbe3944/src/firestore/firestore.ts#L123

[mikelehen]

I believe this is a dupe of #256 which we'll try to address soon-ish. :-)