Validate required variables and support (or show how to use) provider {} HCL section

[spkane]

After local testing we moved our terraform into a Jenkins based workflow, but forgot to set the JIRA_URL in the environment. The apply failed with a very mis-leading error message:

[global] Error: Error applying plan:
[global] 
[global] 1 error(s) occurred:
[global] 
[global] * module.base_account.jira_issue.aws-account-creation-otc: 1 error(s) occurred:
[global] 
[global] * jira_issue.aws-account-creation-otc: unexpected EOF

The provider should really verify that the variables are non-null, non-empty string, and that the URL at least looks like a proper URL, and if not it should provide some useful feedback about the issue.

I'm also not sure if the provider supports the provider HCL block, but if not, it would be useful and an example of how to set it up would also help. I can imagine something like this.

provider "jira" {
  url = "https://example.atlassian.com"
  user = "user@example.com"
}