UX improvements for update plans

[Frassle]

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Issue details

1. Consider renaming --save-plan to --plan-out so it sorts with "plan", or possibly just --out if we don't expect preview to output anything else.
2. It's not clear what extension to use for plan files. Should we say it's JSON in the help text?
3. Should pulumi up --plan x.json indicate somehow that it's using the plan file?
4. It's might feel odd that pulumi up still runs a preview with a plan file. Again should we change the cli text to make it clear that the plan is being validated? Something like rather than "Previewing update (dev)" it can say "Applying plan (dev)", "Validating plan (dev)", or something?
5. Related to above should it explicitly say, "Update preview verified to be consistent with plan x.json" or something.
6. Should the cli print that it's generated and used a plan as part of pulumi up when no explicit plan has been passed in.
7. Do we need an option to turn the auto planning off? --no-plan?
8. Some error messages are not consistent:
   "error: resource X violates plan: properties changed: …"
   "error: create is not allowed by the plan: no steps were expected for this resource"
   First error has URN, second does not; first says "violates plan", second says "not allowed by plan"; perhaps just:
   "error: resource X violates plan: properties changed: …"
   "error: resource X violates plan: create not allowed, no steps were expected for this resource"
   IOW, always prefix with "resource X violates plan."
9. Print a message with a link to more info about update plans when constraints fail. Something like "If you are seeing this, it means something changed between your preview and plan, or there is non-determinism in your Pulumi program. Read more here: https://pulumi.com/"