plugin: Fix an error when evaluating sensitive values

[wata727]

Fixes #1457
See also terraform-linters/tflint-plugin-sdk#174

When evaluating a variable with sensitive = true, TFLint fails with the following error:

Failed to check ruleset; Failed to check `aws_instance_previous_type` rule: value has marks, so it cannot be serialized

This happens because sensitive values are implemented as cty marked values. Marked values will cause an error when encoded in the message pack format to respond to plugins.

https://github.com/terraform-linters/tflint-plugin-sdk/blob/v0.11.0/plugin/plugin2host/server.go#L132
https://github.com/zclconf/go-cty/blob/v1.11.0/cty/msgpack/marshal.go#L45-L47

This behavior works very well with sensitive values and avoids unintentional disclosure unless explicitly unmarked.

In this PR, to prevent unintentional disclosure of sensitive values, return an error and stop evaluation instead of unmarking sensitive values.