Make firestore get_check_exists method work as intended. (GoogleCloudPlatform#3408)

Since [this commit](googleapis/python-firestore@068089e#diff-6f034fb43c430e2af1248c2da03ec41dR428-R458) firestore's client `get` method is not raising `google.cloud.exceptions.NotFound` if there is no such document.

`get_check_exists` function is now working properly - needs to be updated. My proposal is `exists` method (works for me).

Author: artemrys

firestore/cloud-client/snippets.py

@@ -15,7 +15,6 @@
 from time import sleep
 
 from google.cloud import firestore
-import google.cloud.exceptions
 
 
 def quickstart_new_instance():
@@ -217,10 +216,10 @@ def get_check_exists():
     # [START get_check_exists]
     doc_ref = db.collection(u'cities').document(u'SF')
 
-    try:
-        doc = doc_ref.get()
+    doc = doc_ref.get()
+    if doc.exists:
         print(u'Document data: {}'.format(doc.to_dict()))
-    except google.cloud.exceptions.NotFound:
+    else:
         print(u'No such document!')
     # [END get_check_exists]