add better error if credentials are invalid

[megan07]

Fixes hashicorp/terraform-provider-google#10234

ReadPathOrContents assumes if the path doesn't exist, it must be contents. But if the file just isn't where we want it, the code thinks it's the contents and proceeds. Let's verify that the credentials are valid json.

[jkugler]

If it cannot find the file, and tries to parse the file name as JSON, it will throw this error, making the user thing the contents of the file are invalid JSON. Will this give a "file not found" error if the file does not exist?

[megan07]

Hi @jkugler, I understand your concern. Unfortunately, this field allows the convenience of being a path to a file OR an intended value, and the function backend.ReadPathOrContents assumes if the file is not found, that it is the intended value. With this convenience, we lose the precise context of the error.

[jkugler]

Understood. Could the error message be changed to "Could not parse '......' as valid JSON, nor could we find a file by that name." Maybe that would help?

[github-actions]

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