Fix and clarify error message when -plugin-dir can't be saved to working directory

[NiklasRosenstein]

Target Release

1.8.x

Draft CHANGELOG entry

• Fixed and improved error message when -plugin-dir is used with terraform init and the values can't be saved into the workspace directory.

BUG FIXES

• The error message would say -plugin-path when the option is actually -plugin-dir.
• Improved said error message a bit to clarify where that value was going to be saved to.

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[nfagerlund]

Oh. Yeah, thanks for catching this. 👍🏼 It looks like the value provided via -plugin-dir gets stored in internal structures under the name "PluginPath" everywhere, so everyone's always primed to use use the wrong name in UI text. 😓

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