core: fix crash on provider warning

[phinze]

When a provider validation only returns a warning, we were cutting the
evaltree short by returning an error. This is fine during a
walkValidate but was causing trouble during walkPlan and
walkApply.

fixes #2870

[mitchellh]

LGTM. I don't see any problems with this especially if all the other tests pass.

Food for thought (not too much thought): there is a lot of weirdness around validation and computed attributes, and I think I commentd about it in various places and put tests in, but we do stuff like: if the config contains something computed, don't call validate during validate. I'm not sure if this affects that, but worth thinking about.

[phinze]

More food for thought - pretty sure this prevents multiple warnings from properly aggregating (since we're still using the error-passing mechanism to get the warnings out). Will need to revisit.

[ghost]

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