Add error message for importing branch protection

[tibbes]

@bateller reported (as a follow-up on #671) that the import function for github_branch_protection doesn't give a nice error message if the resource doesn't exist on GitHub.

Error: nil entry in ImportState results. This is always a bug with
the resource that is being imported. Please report this as
a bug to Terraform.

This pull request changes the message to:

Error: Could not find a branch protection rule with the pattern 'master'.

The trouble is, I don't know how to add an idiomatic test for this change. Any advice would be much appreciated.

[bateller]

LGTM

Also this closes #597
Disregard, we'll need another PR to fix importing by branch_id

[jcudit]

This looks like it'll fix our issue and tests are passing locally for existing functionality. @tibbes happy to merge as-is but will also leave some guidance on a possible approach to testing if you'd like to follow up there:

• Take a look at the TestStep struct, specifically its ExpectError feature
• When defining test cases, there is the ImportState field that can be used to make a test step perform a terraform import of a resource created from a previous step. Our webhook test does this:

		testCase := func(t *testing.T, mode string) {
			resource.Test(t, resource.TestCase{
				PreCheck:  func() { skipUnlessMode(t, mode) },
				Providers: testAccProviders,
				Steps: []resource.TestStep{
					{
						Config: config,
						Check:  check,
					},
					{
						ResourceName:      "github_organization_webhook.test",
						ImportState:       true,
						ImportStateVerify: true,
					},
				},
			})
		}

[tibbes]

@jcudit, thanks for pointing out ExpectError. That's just what I needed. I've added tests for the error message now.

[tibbes]

(sorry—accidental close of PR)