Fix wrong get_check_exists sample. No exception handling is required anymore (#2429)

* Fix wrong get_check_exists sample

There are no more exceptions when no document found in DocumentReference.get() call.

If the document does not exist at the time of the snapshot is taken, the snapshot’s reference, data, update_time, and create_time attributes will all be None and its exists attribute will be False.

https://googleapis.github.io/google-cloud-python/latest/firestore/document.html#google.cloud.firestore_v1.document.DocumentReference.get

Now issues googleapis/google-cloud-python#4530 and googleapis/google-cloud-python#4531 are resolved

* removed unused exceptions import

Author: limexp

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]