Add error message for rsync not found in image

[cblmemo]

This PR partially fixes #1603 by adding an error message that indicates rsync is not found in the specified image, so that users don't need to dive deep into SkyPilot code to figure out why.

#1641 and #1910 will add cloud-init support for GCP and Azure. After these two are merged, we could have a new PR to add the installation of rsync.

An example of the error message:

(sky-dev) ➜  skypilot git:(fix-rsync-not-exist) ✗ sky launch --cloud gcp --image-id projects/skypilot-375900/global/images/no-rsync-centos
I 07-16 14:41:11 optimizer.py:636] == Optimizer ==
I 07-16 14:41:11 optimizer.py:648] Target: minimizing cost
I 07-16 14:41:11 optimizer.py:659] Estimated cost: $0.4 / hour
I 07-16 14:41:11 optimizer.py:659] 
I 07-16 14:41:11 optimizer.py:733] Considered resources (1 node):
I 07-16 14:41:11 optimizer.py:781] --------------------------------------------------------------------------------------------
I 07-16 14:41:11 optimizer.py:781]  CLOUD   INSTANCE        vCPUs   Mem(GB)   ACCELERATORS   REGION/ZONE   COST ($)   CHOSEN   
I 07-16 14:41:11 optimizer.py:781] --------------------------------------------------------------------------------------------
I 07-16 14:41:11 optimizer.py:781]  GCP     n2-standard-8   8       32        -              us-central1   0.39          ✔     
I 07-16 14:41:11 optimizer.py:781] --------------------------------------------------------------------------------------------
I 07-16 14:41:11 optimizer.py:781] 
Launching a new cluster 'sky-aae9-memory'. Proceed? [Y/n]: 
I 07-16 14:41:20 cloud_vm_ray_backend.py:3884] Creating a new cluster: "sky-aae9-memory" [1x GCP(n2-standard-8, image_id={'us-central1': 'projects/skypilot-375900/global/images/no-rsync-centos'})].
I 07-16 14:41:20 cloud_vm_ray_backend.py:3884] Tip: to reuse an existing cluster, specify --cluster (-c). Run `sky status` to see existing clusters.
I 07-16 14:41:23 cloud_vm_ray_backend.py:1390] To view detailed progress: tail -n100 -f /home/memory/sky_logs/sky-2023-07-16-14-40-38-712846/provision.log
I 07-16 14:41:26 cloud_vm_ray_backend.py:1743] Launching on GCP us-central1 (us-central1-a)
I 07-16 14:42:10 cloud_vm_ray_backend.py:1802] Skipping retry due to `rsync` not found in the specified image.
Clusters
NAME             LAUNCHED     RESOURCES                                                                  STATUS  AUTOSTOP  COMMAND                       
sky-aae9-memory  45 secs ago  1x GCP(n2-standard-8, image_id={'us-central1': 'projects/skypilot-3759...  INIT    -         sky launch --cloud gcp --...  

RuntimeError: rsync is not installed on the specific image. Please install rsync and try again.

Tested (run the relevant ones):

• Code formatting: bash format.sh
• Any manual or new tests for this PR (please specify below)
  • sky launch --cloud gcp --image-id <some image without rsync>
• All smoke tests: pytest tests/test_smoke.py
• Relevant individual smoke tests: pytest tests/test_smoke.py::test_fill_in_the_name
• Backward compatibility tests: bash tests/backward_comaptibility_tests.sh

[Michaelvll]

Thanks for the improvement of the UX @cblmemo! Left two comments.

[Michaelvll]

Thanks for adding this @cblmemo! LGTM.