core: Warn when creating and applying with -target

[apparentlymart]

The documentation for the -target option warns that it's intended for exceptional circumstances only and not for routine use, but that's not a very prominent location for that warning and so some users miss it.

Here we make the warning more prominent by including it directly in the Terraform output when -target is in use. We first warn during planning that the plan might be incomplete, and then warn again after apply concludes and direct the user to run terraform plan to make sure that there are no further changes outstanding. The latter message is intended to reinforce that -target should only be a one-off operation and that you should always run without it soon after to ensure that the workspace is left in a consistent, converged state.

---

During plan, the warning message is:

Warning: Resource targeting is in effect

You are creating a plan with the -target option, which means that the result
of this plan may not represent all of the changes requested by the current
configuration.
		
The -target option is not for routine use, and is provided only for
exceptional situations such as recovering from errors or mistakes, or when
Terraform specifically suggests to use it as part of an error message.

During apply, the warning message is:

Warning: Applied changes may be incomplete

The plan was created with the -target option in effect, so some changes
requested in the configuration may have been ignored and the output values may
not be fully updated. Run the following command to verify that no other
changes are pending:
    terraform plan
	
Note that the -target option is not suitable for routine use, and is provided
only for exceptional situations such as recovering from errors or mistakes, or
when Terraform specifically suggests to use it as part of an error message.

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While implementing this I noticed some places where we missed updating for the fact that ctx.Plan and ctx.Apply now return diagnostics rather than errors, so there are some adjustments for that to ensure that the callers really are treating these warnings as warnings, rather than exiting with an error status.

[mildwonkey]

Very nice!

[ghost]

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